The tag is present in Lintian version 2.109.24. That is the most recent version we know about.
Some packaging files obtain different names when they are copied from source to installation packages. Debhelper sometimes adds *.Debian extensions to NEWS, README and TODO files. That can be confusing.
Debhelper's behavior also depends on the filename.
This source package contains both a file with the proper name and also a file with incorrect name. Please remove the file as indicated.
Please merge all relevant information into the surviving file.