Zitate mit selben Authoren und gleichen Jahr

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@Article{Thomasetal2006b,
  Title                    = {Title},
  Author                   = {Thomas, R. and Webster, J.D. and Rhede, D. and Siefert, W. and Förster, H.J. and Heinrich, W. and Davidson, P.},
  Journal                  = {L},
  Year                     = {2006},
  Number                   = {1--4},
  Pages                    = {137--149},
  Volume                   = {91},
  Owner                    = {Unidentified},
  Timestamp                = {2015.04.27}
}

@InCollection{Thomasetal2006a,
  Title                    = {Title 2},
  Author                   = {Thomas, R. and Webster, J.D. and Davidson, P.},
  Booktitle                = {a book},
  Publisher                = {a publisher},
  Year                     = {2006},
  Editor                   = {Webster, J.D.},
  Pages                    = {189--210},
  Series                   = {Notes},
  Volume                   = {36},
  Owner                    = {Unidentified},
  Timestamp                = {2015.04.27}
}
@Article{VekslerThomas2002,
  Title                    = {Title},
  Author                   = {Veksler, I.V. and Thomas, R.},
  Journal                  = {journal},
  Year                     = {2002},
  Number                   = {6},
  Pages                    = {673--683},
  Volume                   = {143},
  Owner                    = {Unidentified},
  Timestamp                = {2015.04.27}
}
@Article{Meieretal2012,
  Title                    = {Tilte1},
  Author                   = {Meier, K.L. and Schulz, A.U. and Name, W.B. and anothername, E.E.},
  Journal                  = {Journal1},
  Year                     = {2012},
  Number                   = {12},
  Pages                    = {1777--1791},
  Volume                   = {38},
  Timestamp                = {2015.04.27}
}

@Article{Meieretal1997,
  Title                    = {Title2},
  Author                   = {Meier, K.L. and Muller, A.U. and Schulz, W.B. and Name, E.E.},
  Journal                  = {Journal2},
  Year                     = {1997},
  Number                   = {12},
  Pages                    = {1777--1791},
  Volume                   = {38},
  Timestamp                = {2015.04.27}
}

@Article{MeierSchulz1989,
  Title                    = {Title3},
  Author                   = {Meier, K.L. and Schulz, A.U.},
  Journal                  = {Journal},
  Year                     = {1989},
  Number                   = {12},
  Pages                    = {1777--1791},
  Volume                   = {38},
  Timestamp                = {2015.04.27}
}
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\begin{document}

bla \citet{Thomasetal2006a} bla \citet{Thomasetal2006b} bla
\citet{VekslerThomas2002} bla

Es sollte da stehen: bla Thomas et. al (2006a) bla Thomas et al. (2006b) bla
Veksler \& Thomas (2002)

bla \citet{Meieretal1997} bla \citet{Meieretal2012} bla \citet{MeierSchulz1989} bla

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\end{document}