I suggest to use `sed` to collect lines in the hold space to check whether they appeared before:

 sed -n 'H;G;/^\(C([^)]*)\).*\1 *\n/!P'

* `H` appends the current line to the hold space
* `G` appends the hold space with all lines we ever saw to the pattern space
* `C([^)]*)` is one of those `C(…)` patterns, the `^` anchors it to the beginning of the line and it's surrounded by `\(…\)`, so it can be backreferenced as `\1` later. We need `\1 *\n` as pattern, with the newline (after possible whitespaces) to avoid matching the freshly appended line at the end. So the whole pattern `/^\(C([^)]*)\).*\1 *\n/` matches a line with a duplicate `C(…)`, so only if this `!` doesn't match,
* `P`rint everything before the first newline (= without the appended hold space), while default output is suppressed by the `-n` option

Note that depending on you `sed` version and file size, this may fail because over the time, all lines will be in memory.