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.