Retina clusters with the same LHCb ID

In rare cases Retina clustering splits a cluster into two clusters that are centered in the same pixel and have slightly different coordinates (see image).
Being centered in the same pixel they share the same LHCb ID.
Phi sorting and the corresponding pre-condition in the search by triplet contract have been modified within !655 (merged) to take care of this.
If two clusters have the same phi and same LHCb ID they are sorted by x (major) and y (minor).
Having two clusters with the same LHCb ID might affect tracking since there is no more a one-to-one mapping between IDs and hits.
A possible solution to this might be keeping only one of the two clusters that share the same LHCb ID.

FYI @dcampora