Duplicate question

Is there any way to sort of make a “Mirror” duplicate? Because when i duplicate my object its exactly the same as the bit i already had meaning i cant join them together (i am trying to duplicate half a car so i can join the 2 halves together) also the mirror modifier doesn’t work because i cant select the piece that is mirrored without it selecting the rest of the model with it.

Alt-D will create linked duplicates in object mode . Then you just press M and select the axis you want to mirror . And then when you are finished you have to join it to the main model with Ctrl-J …Though you have to separate the part from the rest of the mesh with P before doing this as you can’t create linked duplicates in Edit Mode … Or if you don’t need them to be linked Shift-D then hit M and select the axis you want to mirror along .