I modeled a character with the mirror modifier.
I have applied the mirror modifier so I have a full mesh.
When I UV unwrap I get what you see below.
Be sure you have applied your mirror modifier
Select the whole model, press U then “Reset” or “Project from view” to change the current uv data
Press U then Unwrap
or in case you want to keep what you did but just mirror the UV accordingly :
In the UV Editor, select 1 vertice in what will be the center of the symetry of one of the UV islands, press (edit) SHIFT+S and select “cursor to selected”
Change the Pivot to 2D Cursor in the UV Editor
Now enable this button.
select one of the islands
unselect the button i mentionned
in the 3D view, every faces that correspond to the overlapped island will be highlighted. Unselect one side.
in the UV Editor, click on UV -> Mirror -> X Axis
select the whole model in 3D view to check if the UV is now correct for that island, and it should be.