Incremental tape re-use

["Donald H. Mitchell" <dmitchell%backus.trc.amoco.com@RELAY.CS.NET>]

WARNING: NEW USER QUESTION--

I have a low volume of file changes on my lonely Symbolics and wanted to
minimize tape storage; therefore, I tried using the "leave" value for
the "tape when done" field on the incremental backup thinking that I
should be able to append the next backup at the end of the previous one.
Well, two or three cold boots later, I tried doing an incremental backup
and got the warning that doing so would be hazardous to my tape's life.
Did I miss something or should I infer that "leave" is very temporary?
Is there anyway to do more than one backup to the same tape?