Update of store after adding ADDITIONAL XML DATA FIELDS is not resistant to duplicated id.

[JolkaS]

Steps to reproduce:

1. define minimal XmlDataSource :
   
2. Run job "update stores offers"
   logs: https://gist.github.com/noisy/04a892bc8fdafff00dac305639ea209c
3. modify XmlDataSource by adding ADDITIONAL XML DATA FIELD:
   
4. Run job "update stores offers

Current behavior:
update fail
logs: https://gist.github.com/noisy/b41fb149118152a2bb7ab6adcfbd2fed

Expected behavior:
Offers with duplicated Id should be ignored (like in first update)

[noisy]

Because of lost old revision of XMLfiles, right now I am not able to reproduce this problem :( In case of reoccurrence, please let me know