fdo#63553: sw: fix copy/paste of cross references

If one of aIds or aDestIds contains nSeqNo then adding an identity
mapping for it will actually re-define a previous mapping.  Likely this
can only happen when clipboard document is the source.

(regression from 44f971506c0ed37928c48e55d8007f24b0c43a5f)

(also it looks like 4b0b0a5b3045902d9a6b95510ec30a3027181e29 has subtly
 changed what the "else" branch does when it is erroneously taken -
 results are differently wrong)

Change-Id: I34e2979e45cffeb62ebf3d7b3e5ac6d2afff0c2b