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

 sed -n 'H;G;/^\(C([^)]*)\).*\1 *\n/d;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 nexline (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 we delete that one
• otherwise we Print everything before the first newline (= without the appended hold space), while default output is suppressed by the -n option