Code duplication is usually bad and often quite easy to spot. I suppose that compilers could detect it automatically in easiest cases – they already parse the text and get the intermediate representation that they analyze in various ways – detect suspicious patterns like uninitialized variables, optimize emitted code, etc. I guess they could often detect functionally duplicate code this way as well and account for it while emitting machine code.
Are there C++ compilers that can detect duplicate code and only emit corresponding machine code once instead of for each duplicate in the source text?
Some do, some don’t.
From the LLVM optimization’s page: -mergefunc (MergeFunctions pass, how it works)
The functions are separated in small blocks in the LLVM Intermediate Representation, this optimization pass tries to merge similar blocks. It’s not guaranteed to succeed though.
You’ll find plenty of other optimizations on this page, even though some of them may appear cryptic at first glance.
I would add a note though, that duplicate code isn’t so bad for the compiler / executable, it’s bad from a maintenance point of view, and there is nothing a compiler can do about it.