The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
This paragraph defines an already defined license.
According to the specification, short license names are required to be unique within a single copyright file.
This tag could be raised by something like this:
Files: filea ... Copyright: 2009, ... License: LGPL-2.1 This program is free software; ...
Files: fileb ... Copyright: 2009, ... License: LGPL-2.1 This program is free software; ...
In this case, you redefine LGPL-2.1 license. You should use a stand-alone paragraph or merge the two files (using a single paragraph).
Refer to https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/ for details.