Deadlock in Implementation

After quite a few days debugging finally spotted the problem...

Consider the following case:

1) a1 initiates a proposal with a3 and a4.
2) a2 initiates the same proposal with a3 and a4.
3) a3 responds to a1 with acceptance.
4) a4 responds to a2 with acceptance.

In this situation a3 cannot respond to a2 and a4 cannot respond to a1. Both are stuck.

Now need to think what to do about it!