#48 filterUsersAbleToTransitionFrom shows a user multiple times

[Richo]

If a user can have multiple roles within a proyect, which in our case it is possible, the it is shown in the assignto select box several times.
So modified the code so that an user gets added to the list of users to include in the select box only once.
My patch may not be too elegant but it works.

[Richo]

Patch to correct the filterUsersAbleToTransitionFrom