Fix exception about duplicate entires in SmtInterpolModel if uf(a) and uf(b)
occur in the formulas and a and b have the same value.
Also add unit tests that checks this case.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14931 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c