Activity with duplicate name

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I've just set up an activity for Worcestershire Bat Group (WBG) records. But whenever someone tries to add that activity to 'my activities' or enter a record they get the following message: "The group you are trying to join has a duplicate name with another group so cannot be joined in this way." Someone who tried this showed me a screenshot of what seems to be a very old WBG 'group' (we have not idea when or who set it up) but I can't for the life of me find 'groups' under 'activities', so I can't see the details. And the old group/activity doesn't appear when I search for it using 'browse all activities', nor do I know how to resolve the conflict. Any ideas please? Thanks!