Simple mirroring

Select your model.

Open UV Editor.

Select “Symmetrize UV Util” tab.

Click “Symmetrize”.

Choose direction by moving cursor.

Click to apply.

Now the UV island has been mirrored.

Use a brush

Click “Start Brush”.

Click + Move mouse cursor to mirror a part of the island.

Press “F” key to change radius of the brush.

Right-click to exit brush mode.