Ambiguous name error appears when using distribution lists

A user attempts to send an e-mail to a distribution list (for example, PlanDirect) and Types "PlanDirect," she gets the following:

Ambiguous Name (error dialog) "More than one match was found for name Resourcing."

She is using the 5.04 client. When I tried it on my client (5.09), it works fine -- I type exactly the same case (PlanDirect) that has been used. If I type it all in lowercase, I receive the dialog box. For her, proper case doesn't help. Any ideas?
This is working as intended. The idea is that the system should find all names that start with whatever is typed into the field.

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