You have multiple XML files modifying the same ArtDef (GameItemTypeArtDef). You are effectively repeating the graphics files.
If you for example copied a horse mount, your XML should look like this:
<GameItemTypeArtDef InternalName="BasicHorseMount_ArtDef">
<GameItemTypeModelPack InternalName="BasicHorseMount_Default">
<SupportedUnitModelType>Your New Modeltype</SupportedUnitModelType>
</GameItemTypeModelPack>
</GameItemTypeArtDef>
Note I'm not leaving anything out, that's all you need to add a modeltype to mounts from the /mods/ folder.