The explanation why preimages should not be reused, is as a result of it permits anybody alongside the routing path that has beforehand already seen (and saved) the preimage to say these funds with out forwarding the fee to the meant vacation spot. See additionally Reuse of fee hash in Lightning Community
How would the receiver be capable to distinguish between these two funds?
I am undecided about how the Lightning Node implementations deal with this in follow, however in principle, they need not. Anytime your node receives an HTLC for which you already know the preimage of the hash (both since you generated the preimage, or as a result of the sender included the preimage within the HTLC as per the keysend spec), you may declare the funds dedicated to by that HTLC.
See additionally How do ‘Sphinx’ invoice-less funds on the Lightning Community work?
