Bug: relation markers accept duplicate requirement entries

[haxtibal]

MarkerParser.parse will accept a marker like @relation(REQ-1, REQ-1, scope=function) from source code files. It simply constructs a FunctionRangeMarker with reqs=["REQ-1", "REQ-1"].

This can lead to errors in later processing stages, e.g. when FileTraceabilityIndex.validate_and_resolve does for req_ in function_marker_.reqs: traceability_index.graph_database.create_link(...).

It's rather hard to connect the later error to the original root cause. We should better prevent duplicate reqs entries in first place. Maybe use set instead of list, and print warning to user if duplicate entry is detected.

I added this bug as a reminder to myself, but everybody feel free to fix it :)