There are a number of known duplicates in the collection.
The reason for keeping both is as below:

QSID Countdown
==============
These were released as a set, so keeping the version with the QSID text as well as the original.

Misc\Qsid_Countdown\QSID_01_Close_my_Eyes_Forever.mus  Sean_Fields\Close_my_Eyes_Forever.mus  
Misc\Qsid_Countdown\QSID_02_Wish_you_were_Here.mus     Archmagus_-Rob_Wygand-\Wish_you_were_Here_A.mus
Misc\Qsid_Countdown\QSID_03_Layla.mus                  DC_Holmes\Layla.mus
Misc\Qsid_Countdown\QSID_04_Sweet_Child_of_Mine.mus    BJ_Pools\Sweet_Child_o_Mine_B.mus
Misc\Qsid_Countdown\QSID_05_Yellow_Submarine.mus       DC_Star_and_Cpr\Pictures\Yellow_Submarine.mus
Misc\Qsid_Countdown\QSID_05_Stationary_Ark.mus         Misc\M\Stationary_Ark.mus
Misc\Qsid_Countdown\QSID_06_Mystery_Train.mus          Jerry_Roth_-Dr_J-\Mystery_Train.mus
Misc\Qsid_Countdown\QSID_07_Immigrant_Song.mus         Sadist_-Madman-_-Mike_Madsen-\Immigrant_Song.mus
Misc\Qsid_Countdown\QSID_08_Call_of_Ktulu.mus          Uncle_Slam\Call_of_Ktulu.mus
Misc\Qsid_Countdown\QSID_09_Baby_Elephant_Walk.mus     Lyric\Baby_Elephant_Walk_B.mus
Misc\Qsid_Countdown\QSID_10_Heat_of_the_Moment.mus     Misc\S\Heat_of_the_Moment.mus


Alternative Words
=================
These either have different words or a version with parody words.

Bobbye\Mammas_dont_let_your_Babies_grow_up_to_be_QGuides.mus          Bobbye\Mammas_dont_let_your_Babies_grow_up_to_be_Cowboys.mus
Bobbye\Send_in_the_Clones.mus                                         Bobbye\Send_in_the_Clowns.mus
Datasid_-Troy_Rutter-\Hate_to_See_Cher.mus                            DC_Star_and_Cpr\Hip_to_be_Square.mus
Datasid_-Troy_Rutter-\Its_a_Hell_Hole_after_all.mus                   Nick_Zelinsky\Its_a_Small_World.mus
DC_Star_and_Cpr\Life_on_Quantumlink.mus                               DC_Star_and_Cpr\Where_do_I_begin.mus
Misc\Anon\My_Bologna.mus                                              Lyric\My_Sharona.mus
Misc\E\Walk_like_a_Grandmaw.mus                                       Nick_Zelinsky\Walk_like_an_Egyptian.mus
Misc\L\The_Day_the_Bell_System_Died.mus                               Jim_Masterson\American_Pie.mus
Misc\W\Club_called_ICPUG.mus                                          Dean_Schold\House_of_the_Rising_Sun.mus
Nick_Zelinsky\This_Old_Q-Link.mus                                     Nick_Zelinsky\This_Old_Guitar.mus
Sean_Fields\This_Songs_just_Six_Words_Long.mus                        DC_Holmes\Got_my_Mind_set_on_you.mus
Spiecher\Rainbow_Connection_A.mus                                     Spiecher\Rainbow_Connection_B.mus
Stewart_Meyers\Along_the_Grand_Canal.mus                              DC_Star_and_Cpr\What_Child_is_this.mus
Stewart_Meyers\Babylon_Pie.mus                                        Jim_Masterson\American_Pie_A.mus
Stewart_Meyers\Ballad_of_Cyrano_Jones.mus                             Nick_Zelinsky\Ballad_of_Jed_Clampett_Beverly_Hillbillies.mus
Stewart_Meyers\Ben_Sisko.mus                                          Bobbye\Fernando_A.mus
Stewart_Meyers\Bill_Gates_Song.mus                                    Misc\Anon\Christmas_Song.mus
Stewart_Meyers\Capitol_Steps_Song.mus                                 Les_Herrman\Impossible_Dream_C.mus
Stewart_Meyers\Chaostime_in_the_City.mus                              Misc\Data_Dugout_BBS\Silver_Bells.mus
Stewart_Meyers\Chipwonk_Song_(BAD).mus                                Dean_Schold\Chipmunk_Song.mus
Stewart_Meyers\Christmas_Time_is_Coming_Around.mus                    Etienne\Santa_Claus_is_Coming_to_Town.mus
Stewart_Meyers\Clinton_Bash.mus                                       Spiecher\Monster_Mash.mus
Stewart_Meyers\Coward_of_the_Daleks.mus                               Tom_B24\Coward_of_the_County.mus
Stewart_Meyers\Ding_Fries_are_Done.mus                                Bob_Huffman\Carol_of_the_Bells.mus
Stewart_Meyers\Doctor_the_Eighth.mus                                  Nick_Zelinsky\Im_Henry_the_VIII_I_am.mus
Stewart_Meyers\Doctor_the_Magic_Time_Lord.mus                         Nick_Zelinsky\Puff_the_Magic_Dragon.mus
Stewart_Meyers\Eat_It.mus                                             George_Bounacos\Beat_it.mus
Stewart_Meyers\Frosty_the_Dopeman.mus                                 Les_Herrman\Frosty_the_Snowman_B.mus
Stewart_Meyers\Grandma_got_Hung_Over_Drinking_Rainier.mus             Nick_Zelinsky\Grandma_got_Runover_by_a_Reindeer.mus
Stewart_Meyers\Happily_Addicted_to_the_Web.mus                        Misc\R\Winter_Wonderland.mus
Stewart_Meyers\Hello_Mudduh_Hello_Faddah.mus                          Bobbye\Hello_Mudduh.mus
Stewart_Meyers\I_came_upom_a_Roadkill_Deer.mus                        Bob_Huffman\It_came_upon_the_Midnight_Clear.mus
Stewart_Meyers\Im_Looking_over_my_Dead_Dog_Rover.mus                  NeilH4\Im_Looking_over_a_4_Leaf_Clover.mus
Stewart_Meyers\Jadzia_Dax_Tribute_Song.mus                            DC_Star_and_Cpr\Candle_in_the_Wind.mus
Stewart_Meyers\Last_of_the_Babylon_Stations.mus                       Lyric\Wreck_of_the_Edmund_Fitzgerald.mus
Stewart_Meyers\Newt_Gingrich_is_Running.mus                           Etienne\Santa_Claus_is_Coming_to_Town.mus
Stewart_Meyers\Oh_What_a_Show.mus                                     DC_Starr\Oh_what_a_Night.mus
Stewart_Meyers\Rudolph_the_Brown_Nosed_Reindeer.mus                   Misc\Anon\Rudolph_the_Red_Nosed_Reindeer.mus
Stewart_Meyers\Saturday.mus                                           Ml_Surface\Yesterday.mus
Stewart_Meyers\Secret_Asian_Man.mus                                   Nick_Zelinsky\Secret_Agent_Man.mus
Stewart_Meyers\Spinball_Wizard.mus                                    Michael_French\Pinball_Wizard.mus
Stewart_Meyers\Subpoenaville.mus                                      Lyric\Margaritaville.mus
Stewart_Meyers\Teddy_the_Red_Nosed_Senator.mus                        Misc\Anon\Rudolph_the_Red_Nosed_Reindeer.mus
Stewart_Meyers\Thats_Science_Fiction.mus                              Ernest_True\Thats_Entertainment.mus
Stewart_Meyers\The_Master.mus                                         Paul_Labeaud\Dentist.mus
Stewart_Meyers\Voyager_Bunch.mus                                      Nick_Zelinsky\Brady_Bunch.mus
Stewart_Meyers\We_want_our_DS9.mus                                    Dean_Schold\Money_for_Nothing.mus
Stewart_Meyers\Webster_Bernie_and_Mike.mus                            Nick_Zelinsky\Abraham_Martin_and_John_A.mus
Stewart_Meyers\Wreck_the_Malls.mus                                    Misc\Anon\Deck_the_Halls_with_Boughs_of_Holly.mus
Stewart_Meyers\You_can_Sue.mus                                        Misc\Anon\Jingle_Bells_A.mus
The_Mad_Sidder\Mammas_dont_let_Your_Babies_Grow_up_to_be_No-Daks.mus  Ron_Doss\Mammas_dont_let_your_Babies_Grow_Up_to_be_Cowboys.mus
The_Mad_Sidder\Off_the_Road_Again.mus                                 Tom_B24\On_the_Road_Again_C.mus
The_Mad_Sidder\The_Rambler.mus                                        The_Mad_Sidder\The_Gambler.mus
Whole_Note\Sid_I_Blew.mus                                             Whole_Note\Song_Sung_Blue.mus


Normal and Ext Words
====================
One version has normal words and one version has extended words.

DC_and_Bert\Right_here_Waiting.mus              DC_Star\Right_Here_Waiting.mus
Lyndie\Stranded_A.mus                           Lyndie\Stranded_B.mus
Nick_Zelinsky\Everybody_have_Fun_Tonight_A.mus  Nick_Zelinsky\Everybody_have_Fun_Tonight_B.mus
Nick_Zelinsky\Muppet_Show_A.mus                 Nick_Zelinsky\Muppet_Show_B.mus


Additional Stereo
=================
When there are two versions, one mono and one stereo, with the mus file the same in both, then usually only the stereo version is kept.
However, when the stereo file is an addition by a different author than the original mono then both versions have been kept.

Kenneth_Barsky\Battlestar_Galactica.mus+str   Brian_M_Szepatowski\Battlestar_Galactica_A.mus       
Jerry_Roth_-Dr_J-\Long_Tall_Sally_B.mus+str   Jerry_Roth_-Dr_J-\Long_Tall_Sally_A.mus


Others to Keep
==============
These are where I've decided to keep both for various reasons.

Since they are part of a set, will keep both. Also III is mono and IV is stereo.
DC_Starr\Star_Trek_III_Prologue.mus                   DC_Starr\Star_Trek_IV_Prologue.mus

Both versions have different Ext wds.
DC_Star_and_Cpr\Love_in_the_First_Degree_A.mus        DC_Star_and_Cpr\Love_in_the_First_Degree_B.mus

Both versions have different Ext wds.
Ed_Grimely\Grandma_got_Runover_by_a_Reindeer_A.mus    Ed_Grimely\Grandma_got_Runover_by_a_Reindeer_B.mus

The 5 lines of text is different in these 3 identical tunes.
Harry_Bratt\HB\More_Shit1.mus    Harry_Bratt\HB\More_Shit2.mus    Harry_Bratt\HB\More_Text.mus

Deliberate mono and stereo versions that are part of a set of 3 tunes. Only this one has identical mus file, so kept both.
Syl\Violin_Concerto_in_A_Minor_No2_Andante_Mono.mus   Syl\Violin_Concerto_in_A_Minor_No2_Andante_Stereo.mus


The Rest
========
These are where I can't decide which is the best to keep.  Would aim to keep the one by the original author, when the authors differ.

Dale_Clark\Picnic.mus                     Ettie_Wells\Picnic.mus
Dave_Marquis\Ciribiribin_B.mus            Kenneth_Barsky\Go_Down_Moses.mus
Dave_Marquis\Pizzicato_Polka_A.mus        Frank_Higgins\Pizzicato_Polka.mus
Dave_Marquis\Tritsch_Tratsch_Polka_B.mus  Frank_Higgins\Tritsch_Tratsch_Polka.mus
Dave_S2\76_Trombones.mus                  Dave_Stewart\76_Trombones.mus
Eugenes1\Any_Day_Now.mus                  Gene\Any_Day_Now.mus
Eugenes1\Cajun_Moon.mus                   Gene\Cajun_Moon.mus
Eugenes1\Downtown_Train.mus               Gene\Downtown_Train.mus
Eugenes1\I_Loved_em_Every_One.mus         Gene\I_Loved_em_Every_One.mus
Eugenes1\Lost_in_the_Fifties_Tonight.mus  Gene\Lost_in_the_Fifties_Tonight.mus
Eugenes1\Maggie_May.mus                   Gene\Maggie_May.mus
Eugenes1\Nightshift.mus                   Gene\Nightshift.mus
Elgar\Here_comes_Santa_Claus.mus          Sammy_Snake\Here_comes_Santa_Claus.mus
FatalCharm\Jingle_Bell_Rock.mus           Sammy_Snake\Jingle_Bell_Rock.mus
Jari_A_Kaarima\Boogie_Oogie_Oogie.mus     Roth_Ladoad\Boogie_Oogie_Oogie.mus
Laura\America.mus                         Lyric\America.mus
Laura\Ghosts.mus                          Lyric\Ghosts.mus
Misc\M\Take_on_me.mus                     Misc\O\Take_on_Me.mus
Robert_Willis\Tambourin.mus               Z_Axis\Tambourin.mus
Roth\Helena_Polka.mus                     Roth_Ladoad\Helena_Polka.mus
Whole_Note\Chantilly_Lace.mus             Whole_Note\Jam_Session.mus
