No PRACK sent in parallel forking scenario

[MassimoMessore]

I'm facing an issue when using Doubango in the following scenario:

1. The UAC running Doubango sends an INVITE request and the proxy forks it to create two different dialogs.
2. The UAC receives two different 180 responses that need to be both PRACKed (they have the same RSeq value but two different to-tags).

                UAS
              /
             / 180 to-tag=a
UAC--->Proxy
             \ 180 to-tag=b
              \
               UAS

In the current implementation, when receiving the second 180 response, I got the warning at: https://github.com/DoubangoTelecom/doubango/blob/master/tinySIP/src/dialogs/tsip_dialog_invite.c#L1286 and no PRACK is sent back to the second dialog.

Could you please fix this?

Thanks in advance.