I want to ask that should i overlap the UV’s which are same, i.e. in case of wheels,
& in other parts of the cart, or i arrange every single UV separately.
As its seen its a game model. So i have to unwrap it according to it. Also I have to texture it.

If the wheels all share the same map, ther is no reason they should not share the same mapping too. Or more to the point, there is no reason to map them separately, unless you want them different to each other, for exapmle, varied dirt.
Typically you would model and map one wheel and copy it to avoid repetetive modeling/mapping tasks.

I know this reply is completely unrelated to your post, but I wanted to ask:

Why all of the extra polygons on the top of the cart?  It looks as though it’s a flat surface with 6 sides.  You could simply connect the two middle verts to create 2 polygons on the top instead of, I believe, 8 or 9.