Did you try increasing the time of membrane washing steps?

i wash the membrane 4 times 5 min interval after blocking after primary incubation and after secondary incubation

For how long do you carry out your blocking step. Do you block with non-fat dry milk or do you use BSA ? 

yes i use 5% non fat dry milk for blocking ( 1 hour )

I guess I would need some clarification before providing any feedback that is helpful, Amit.  You say dimers, but is this a non-denaturing gel?  If not and this is a denaturing gel, then you won't see dimers of your protein.  Do you mean doublets?  If this is a denaturing gel, then it is wholly possible that these bands you see are modified forms of your protein of interest... or degradation products.