when your flying around in your modded textured dagger, go fly around and look for npc flying the regular daggers, you should notice they have the exact same texture as the modded ship your flying. Trust me on this, they will. Freelancer reads in the data per material id of the ships mesh. ie, in the cmp, in the vmesh, per each component of the mesh it has a material id for a specific material. By copying the daggers cmp and changing it's name, that's not changing the material id with it's mesh, hence, when freelancer loads the data up for the material id for your modded dagger, when it comes up to load the data for the regular one it won't reload it because it thinks it already has cause the two have the same material id number. It doesn't matter if your have the two use different .mats, when it reads in one, it will use that texture for everything using that material id.
This is the same deal with new icons as well, you can't just copy an old icon and put a new texture in it, well, you can, and it will work, but it will show either one icon or another for every item which used that base .3dl regardless of name changes. There is a long tut on icon making which you can use to see how to use the freelancer crc calculator for your ship and the mesh data. Cause icons are infact 3d models just like the ships, just much simpler, but same contexts behind them.