Why do I get "Account Already Exists" notice when attempting to accept a Site Invite on Engage?

If you have an existing Engage account, DO NOT select Create New Account, select Sign In.
Only select Create New Account, if you do not have an existing Engage account.
A single Engage account can support multiple Engage sites.
 

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INFORMATION PROVIDED THROUGH THIS SITE IS PROVIDED TO YOU AS IS WITHOUT ANY EXPRESS REPRESENTATIONS OR WARRANTIES OF ANY KIND, AND WE MAKE NO REPRESENTATION OR WARRANTY THAT THIS SITE(OR ANY INFORMATION PROVIDED IN RESPONSE TO YOUR INQUIRY), WILL BE ACCURATE, COMPLETE, OR ERROR-FREE.

YOU AGREE THAT YOU MUST EVALUATE ALL INFORMATION AND RESPONSES, AND THAT YOU BEAR ALL RISKS ASSOCIATED WITH, THE USE OF THIS SITE, INCLUDING ANY RELIANCE ON THE ACCURACY,COMPLETENESS, OR USEFULNESS OF ANY INFORMATION OR MATERIALS MADE AVAILABLE THROUGH THIS SITE.