fixup! make more clear what counts as the second key

Author: jonasnick

doc/musig-spec.mediawiki

@@ -25,7 +25,7 @@ This document is licensed under the 2-clause BSD license.
 * The output of the ''KeyAgg'' algorithm depends on the order of the input public keys.
 * It is possible to sort the public keys with the ''KeySort'' algorithm before key aggregation to ensure the same output, independent of the (initial) order.
 * The KeyAgg coefficient is computed by hashing the key instead of key index. Otherwise, if the pubkey list gets sorted, the signer needs to translate between key indices pre- and post-sorting.
-* The second unique key in the pubkey list given to ''KeyAgg'' gets the constant KeyAgg coefficient 1 which saves an exponentiation (see the MuSig2* appendix in the [https://eprint.iacr.org/2020/1261 MuSig2 paper]).
+* The second unique key in the pubkey list given to ''KeyAgg'' (as well as any keys identical to this key) gets the constant KeyAgg coefficient 1 which saves an exponentiation (see the MuSig2* appendix in the [https://eprint.iacr.org/2020/1261 MuSig2 paper]).
 * The public key inputs are serialized using x-only (32 byte) instead of compressed (33 byte) serialization. The reason for this is that as x-only keys are becoming more common, the full key may not be available.
 * The public nonces are serialized in compressed format (33 bytes). We accept the small overhead compared to x-only serialization to avoid complicating the specification.