Resource shows twice

[Dave]

I have one user "Dave" in my users list ("Manage Users and Groups"). I have
one "Dave" in Resource Center. When I click on "Dave" and choose "View
Availability" it is split twice into "Dave" and "Dave Lowndes". When I check
out "Dave" using Tools > Enterprise Options > Open Enterprise Resource Pool
I get just one row.

Why? How can I fix this?

Thanks!

Dave