Bluetooth: Audio: Scan Delegator: Guard for duplication in add_source

Added a check for duplication of add source operation in the Scan
Delagator. BAP 6.5.4 states that an assistant may not perform add
source operation if it results in a duplication of a source. While
this is not mentioned for the Scan Delegator, the spirit of the spec.
must be that no duplications can exists in the recv_state, which is
what this commit fixes.

Signed-off-by: Fredrik Danebjer <frdn@demant.com>