Ist es möglich, doppelte IP-Adressen für zwei serielle Schnittstellen zu haben, die zum selben Router gehören??

Ja, duplicate ip addresses are allowed on serial interfaces. It is a more efficient way of bundling links together (ie. MLPPP) and also a better way to preserve address space. Change the encapsulation from the default HDLC to PPP in order to assign duplicate ip addresses.