Hi @Steph in Vero Beach ,
Welcome to our forum!
Is your shared mailbox based on Exchange online or premises?
As I know, if it is Exchange online, when there are too many designated users concurrently accessing a shared mailbox (no more than 25 is recommended), they may intermittently fail to connect to this mailbox or have inconsistencies like messages being duplicated in the outbox. References: About shared mailboxes.
If it is Exchange premises, please refer to this article: Managed Store Limits in Exchange Server.
Hope to help you!
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