duplicate replacements occur if the search string is included in the replace string

[mrv20]

If the search string also matches the replacement string, the replacement is performed more than once on a line.

For example, if a file contains the line:

`ifndef ID_REGS_TEST_SV

running case-keep-replace from id_regs_test to top_id_regs_test will transform the line to:

`ifndef TOP_TOP_ID_REGS_TEST_SV

instead of the expected:

`ifndef TOP_ID_REGS_TEST_SV