Mirror modifer: Mirror without duplicating?

I have an object that is a linked duplicate of another object (so that I can get by with only UVing one object). Now, I need that linked duplicate to actually be a mirror image of the first. For this reason, I would like to use a mirror modifier rather than mirroring it in Edit Mode.

However, I can’t figure out how to use the mirror modifier and keep only the mirror image. I’m assuming it has to do with the Flip settings, but I can’t get those to do anything. But what I really need is an object that is mirrored, not an object AND the mirror, but no matter what I do with the mirror modifier, I still have both copies.

Any advice would be great. Thank you.

Hi, just apply the modifier and in edit mode select all the vertices of the object you don’t need and delete them.

If you place the origin beside the object and check the bisect option you should get what you want.