Merge request shows old commits

When we raise Merge Request in gitlab GUI, it shows up several changes which are already present in target branch.

Version we use : GitLab 7.7.2

is it a bug?

Example:
Master -> M

Branch -> B1 and B2

Few changes done in B1, merge request B1 to M is just fine …just shows valid changes.

But, B1 to B2, merge request shows up many changes which are already present in B2 and they were merged previously on B2.

How to solve this?

Thanks in Advance!