[JENKINS-17332] Allow sign up with username that exist because of SCM people.

core/src/main/java/hudson/security/HudsonPrivateSecurityRealm.java

@@ -310,6 +310,8 @@ public class HudsonPrivateSecurityRealm extends AbstractPasswordBasedSecurityRea
         else {
             User user = User.get(si.username, false);
             if (null != user)
+                // Allow sign up. SCM people has no such property.
+                if (user.getProperty(Details.class) != null)
                     si.errorMessage = Messages.HudsonPrivateSecurityRealm_CreateAccount_UserNameAlreadyTaken();
         }