cr:7.0.5b

If an effect would add a card on a chain link as a defending card, but the card is already defending on that chain link or the card cannot become a defending card due to another effect, the effect fails, no defend event occurs, and the card does not move zones.

Example: Amulet of Havencall has the text “Defense Reaction – Destroy this: Search your deck for a Rally the Rearguard, add it to this chain link as a defending card, then shuffle.” If Rally the Rearguard cannot be added as a defending card, it remains in the deck and the deck is shuffled.

Example: Quickdodge Flexors has the text “Defense Reaction – {r}: Add this to the active chain link as a defending card. It has 2 base {d} this chain link.” If Quickdodge Flexors is already defending card on the active chain link, then its ability is activated, it will remain a defending card on that chain link, no defend event will occur, but its base {d} will still be set to 2.