Run time error 35602

I received this message after trying to place a "thing" on my map. Does anybody know what causes it? How to fix it?

"Run-time error "35602"
"Key is not unique in collection"

I was using a eddited config file.
Here is what I added to it.

monsters
{
color = 12; // Light Red
arrow = 1;
title = "Monsters";
width = 20;
sort = 1;
height = 56;
hangs = 0;
blocking = 2;
error = 2;

3246
{
title "Duke Nukem";
width = 18;
sprite = "DUKEA1";
height = 77;
}


3247
{
title = "Sentry Bot";
width = 16;
sprite = "AGUNA1";
height = 16;
hangs = 1;
}


3245
{
title = "Pig Cop";
width = 33;
sprite = "PIG1A1";
height = 79;
}


3244
{
title = "Agrail";
width = 38;
sprite "HOR1A1";
height = 91;
}


3231
{
title = "Apache";
width = 144;
sprite = "APAHA1";
height = 92;
}


3229
{
title = "Sargant";
width = 22;
sprite = "SOL1E3";
height = 64;
}


3219
{
title = "Slave";
width = 21;
sprite = "SLAVA1";
height 60;
}


3217
{
title = "Head Crab";
width = 23;
sprite = "HDCRA1";
height = 36;
}


3215
{
title = "Infantry";
width = 22;
sprite = "SOLDA1";
height = 64;
}


3106
{
title = "Dragon Familiar";
width = 16;
sprite = "DFAMA1";
height = 33;
}


3128
{
title = "Fire Wyvern";
width = 28;
sprite = "WYVNA1";
height = 35;
}


3103
{
title = "Stone Imp";
width = 20;
sprite = "GIMPA1";
height = 56;
}


3212
{
title = "Cyborg";
width = 22;
sprite = "GMANA1";
height = 55;
}


3204
{
title = "Chainsaw Zombie";
width = 20;
sprite = "CHANA1";
height = 56;
}


3004
{
title = "Former Human";
sprite = "POSSA2A8";
}


9
{
title = "Former Sergeant";
sprite = "SPOSA2A8";
}


3001
{
title = "Imp";
sprite = "TROOA2A8";
}


65
{
title = "Chaingunner";
sprite = "CPOSA2";
}


3002
{
title = "Demon";
width = 30;
sprite = "SARGA2A8";
}


58
{
title = "Spectre";
width = 30;
sprite = "SARGA2A8";
}


3006
{
title = "Lost Soul";
width = 16;
sprite = "SKULA8A2";
}


3005
{
title = "Cacodemon";
width = 31;
sprite = "HEADA2A8";
}


69
{
title = "Hell Knight";
width = 24;
sprite = "BOS2A2C8";
height = 64;
}


3003
{
title = "Baron of Hell";
width = 24;
sprite = "BOSSA2A8";
height = 64;
}


68
{
title = "Arachnotron";
width = 64;
sprite = "BSPIA2A8";
height = 64;
}


71
{
title = "Pain Elemental";
width = 31;
sprite = "PAINA2A8";
}


66
{
title = "Revenant";
sprite = "SKELA2D8";
}


67
{
title = "Mancubus";
width = 48;
sprite = "FATTC2C8";
height = 64;
}


64
{
title = "Archvile";
sprite = "VILEA2D8";
}


16
{
title = "Cyberdemon";
width = 40;
sprite = "CYBRA2";
height = 110;
}


7
{
title = "Spider Mastermind";
width = 128;
sprite = "SPIDA2A8";
height = 100;
}


84
{
title = "Wolfenstein SS";
sprite = "SSWVA2";
}


72
{
title = "Commander Keen";
width = 16;
sprite = "KEENA0";
height = 72;
hangs = 1;
}


88
{
title = "Icon of Sin";
width = 16;
sprite = "BBRNA0";
height = 16;
}


89
{
title = "Monsters Spawner";
sprite = "BOSFB0";
height = 32;
}


87
{
title = "Monsters Target";
height = 32;
}


9050
{
title = "Arachnotron (stealth)";
width = 64;
sprite = "BSPIA2A8";
height = 64;
}


9051
{
title = "Archvile (stealth)";
width = 20;
sprite = "VILEA2D8";
}


9052
{
title = "Baron of Hell (stealth)";
width = 24;
sprite = "BOSSA2A8";
height = 64;
}


9053
{
title = "Cacodemon (stealth)";
width = 31;
sprite = "HEADA2A8";
}


9054
{
title = "Chaingunner (stealth)";
sprite = "CPOSA2";
}


9055
{
title = "Demon (stealth)";
width = 30;
sprite = "SARGA2A8";
}


9056
{
title = "Hell Knight (stealth)";
width = 24;
sprite = "BOS2A2C8";
height = 64;
}


9057
{
title = "Imp (stealth)";
sprite = "TROOA2A8";
}


9058
{
title = "Mancubus (stealth)";
width = 48;
sprite = "FATTC2C8";
height = 64;
}


9059
{
title = "Revenant (stealth)";
sprite = "SKELA2D8";
}


9060
{
title = "Former Sergeant (stealth)";
sprite = "SPOSA2A8";
}


9061
{
title = "Former Human (stealth)";
sprite = "POSSA2A8";
}
}

by "Edited" do you mean you changed the original one, or was it copyed and renamed?

I edited the original Zdoom in Hexen format config. I had eddited it before and it had worked fine.

Maybe you got duplicated thing ids? Try to remove all your new things and then add one at a time and see when the error occurs.

Thanks. I did have two thing ids duplicated.