There is a global restriction (listed under the relics section) stating that they are unique within the army:
The sacred artefacts of the Adeptus Astartes are items of incredible rarity. Only one of each of the following items may be chosen per army – there is only one of each of these items in the galaxy.
However, you're still bounded by the rules for purchasing items according to your army list. In the Space Marines Wargear list, each section specifies what must be traded out when purchasing a piece of equipment. Most sections specify that they can trade either weapon with an and/or clause. However, the subsection for relics contains the line Brandon quoted on page 159:
Only one of each Chapter Relic may be taken per army. A model can replace one weapon with one of the following:
It's also worth noting that special weapons include a similar clause (just an or - not and/or), meaning the 4 flamer / 4 melta command squad would no longer be legal.