This metric measures the number of lines in a file that are contained within a block that is duplicated elsewhere. These lines may include code, comments and whitespace, and the duplicate block may be in this file or in another file.

A file that contains many lines that are duplicated within the code base is problematic for a number of reasons.

Refactor files with lots of duplicated code to extract the common code into a shared library or module.

Wikipedia: Duplicate code.

M. Fowler, Refactoring. Addison-Wesley, 1999.