SQL Query Help on EV server

Hi

I'm getting Event id 3333 (duplicate entries for mailbox) errors for our Journal Mailbox. I've run the following command and confirmed this to be the case:

select * from ExchangeMailboxEntry where LegacyMbxDN = '<Mailbox>'

I get two entries returned with MbxIndentity of '18' and '19'

I can see from the size of the mailboxes that number '19' is the invalid one (zero size etc) and so I want to delete this entry. what command should I run to delete this entry? I tried:

delete from dbo.ExchangeMailboxEntry Where MbxIdentity = 19        

 But that returns the following:

Is that because this is\was a journaling target and needs to be removed from other locations first?

Any help\commands would be greatly appreciated.

kind regards