When one code in target has the same id as one code in source, they must be "corresponding" codes. But we do not validate this because "corresponding" has no associated constraints in the specification.
As a result something like
<pc id='1' canCopy='no'> in source with
<pc id='1' canCopy='no'> in target is seen as valid. But it is wrong in the spirit of the specification.
Note that not all codes must be the same, for example dataRef could have different values, but would need to point to <data> elements with identical content. And even that is possibly not quite true, as mixed representation is allowed, and one code in source could use dataRef while the corresponding code in target could not have dataRef.
Initial thread is here: https://lists.oasis-open.org/archives/xliff/201506/msg00028.html