This is caused by duplicate instances of a constituent in a household group, and when that group member has revenue associated with them. If you go to the Members tab on the household constituent and you see a name appearing more than once, and confirm that each one is the same constituent, then delete the duplicate member instances.

Note:  this error will only occur when the group member is a duplicate and has revenue associated with them. Look at "Total Revenue" on the Summary tab for that constituent:  if it is $0 then there is no revenue associated with them (no error on group constituent). If it is not $0, you will get the error. However, having duplicate group members is bad data, so it is best to remove any you find, even if they are not causing the error.

It is unclear how this happens, but for TA sync clients, the duplicates are ones that are currently not associated with the sync and are only in CRM. Only the CRM specific ones should be removed - the ones tied to the sync should be left. If you cannot determine which this is, or if there are too many to handle manually, contact TA Support for assistance. Support can assist with removing all instances of duplicates that are not associated with the sync.