Added code to prevent adding user with duplicate UserID's

PROMS/VEPROMS User Interface/frmManageUser.cs

@@ -40,6 +40,11 @@ namespace VEPROMS
 				MessageBox.Show("A blank UserID is an invalid UserID");
 				return;
 			}
+			if (UserInfo.GetByUserID(_MyUser.UserID) != null)
+			{
+				MessageBox.Show(string.Format("A user already exists with the UserID '{0}'", _MyUser.UserID));
+				return;
+			}
 			_MyUser.Save();
 			this.DialogResult = DialogResult.OK;
 		}