Fixed nit from PR review; explicitly forbid repeated key indexes in musig()

Author: bigspider

bip-0388.mediawiki

@@ -151,7 +151,7 @@ See [[bip-0379.md|BIP-379]] for a precise specification of all the valid miniscr
 
 
 <tt>KEY</tt> expressions consist of
-* a <tt>KP</tt> expressions
+* a <tt>KP</tt> expression
 * ''always'' followed by either:
 ** the string  <tt>/**</tt>, or
 ** a string of the form <tt>/<NUM;NUM>/*</tt>, for two distinct decimal numbers <tt>NUM</tt> representing unhardened derivations, or
@@ -198,6 +198,8 @@ The public keys obtained by deserializing elements of the key information vector
 
 If two <tt>KEY</tt> are <tt>KP/<M;N>/*</tt> and <tt>KP/<P;Q>/*</tt> for the same key placeholder <tt>KP</tt>, then the sets <tt>{M, N}</tt> and <tt>{P, Q}</tt> must be disjoint. Two <tt>musig</tt> key placeholders are the same if they have exactly the same set of key indexes (regardless of the order).
 
+Repeated <tt>KI</tt> expressions are not allowed inside a <tt>musig</tt> placeholder.
+
 The key information vector should be ordered so that placeholder <tt>@i</tt> never appears for the first time before an occurrence of <tt>@j</tt>  for some <tt>j < i</tt>; for example, the first placeholder is always <tt>@0</tt>, the next one is <tt>@1</tt>, etc.
 
 === Descriptor derivation ===