some OCL messages are displayed multiple times

see email from Aurélie dated 2019-10-14

mentioned in issue riseclipse-metamodel-scl2003#63 (closed)

created merge request !29 (merged) to address this issue

mentioned in merge request !29 (merged)

mentioned in commit 95f5c5f3

closed via merge request !29 (merged)

mentioned in commit 6765e277