Might be considered a bug but posting here since it's an easy fix and (probably) nothing gamebreaking.
Snippet below is from the original/unedited cfgspawnabletypes file (v1.13). Both handguards are seen as separate attachments, making it possible for the game to try to attach them both.
<type name="MP5K">
<attachments chance="0.05">
<item name="MP5_RailHndgrd" chance="1.00" />
</attachments>
<attachments chance="1.00">
<item name="MP5_PlasticHndgrd" chance="1.00" />
</attachments>
To fix this, replace the above part with this.
<type name="MP5K">
<attachments chance="1.00">
<item name="MP5_RailHndgrd" chance="0.05" />
<item name="MP5_PlasticHndgrd" chance="1.00" />
</attachments>
