I looked into the SDPops module and also the SDP core parser. Furthermore I read the relevant RFCs more closely.

It seems that this particular situation (having two ftmp for one media/codec) is not valid.

RFC 4566:
6. SDP Attributes
[...]
a=fmtp:

     This attribute allows parameters that are specific to a
     particular format to be conveyed in a way that SDP does not
     have to understand them.  The format must be one of the formats
     specified for the media.  Format-specific parameters may be any
     set of parameters required to be conveyed by SDP and given
     unchanged to the media tool that will use this format.  At most
     one instance of this attribute is allowed for each format.

RFC 5939:
3.13.1. The "rtpmap" and "fmtp" Attributes
[...]
An "fmtp" attribute capability in a
potential configuration for a media format that already has an "fmtp"
attribute in the actual configuration may lead to multiple fmtp
format parameters for that media format and that is not allowed by
SDP [RFC4566]. The delete-attributes MUST be used to ensure that
there are not multiple "fmtp" attributes for a given media format in
a media description.

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