eMoflon::IBeX 2.0: "Pattern/rule `xy` must not be declared more than once" (validator bug)

[maxkratz]

Related to #424.

Sometimes, the validator shows the message given in the title of this issue but the pattern/rule is not specified multiple times.

Deleting the Eclipse error resolves the issue but cleaning and re-building the whole workspace brings it back (sometimes but not always).