While looking at #17619 (closed) I noticed that SpecConstr appears to be producing more than one specialisation/rule for each call pattern. Specifically, when looking at output from this trace:

@@ -1738,7 +1738,8 @@ spec_one env fn arg_bndrs body (call_pat@(qvars, pats), rule_number)
               rule       = mkRule this_mod True {- Auto -} True {- Local -}
                                   rule_name inline_act fn_name qvars pats rule_rhs
                            -- See Note [Transfer activation]
-        ; return (spec_usg, OS { os_pat = call_pat, os_rule = rule
+        ; pprTrace "sc-rule" (ppr rule) $
+          return (spec_usg, OS { os_pat = call_pat, os_rule = rule
                                , os_id = spec_id
                                , os_rhs = spec_rhs }) }
 

I noticed three distinct rules emitted of the form (up to alpha renaming):

   1 sc-rule                                                                                                                                                                
7464   "SC:$weerr0" [2]                                                                                                                                                     
   1   ┊ ┊ forall (sc_s3W0 :: Int#)                                                                                                                                         
   2   ┊ ┊ ┊ ┊ ┊ ┊(sc_s3W1 :: Int#)                                                                                                                                         
   3   ┊ ┊ ┊ ┊ ┊ ┊(sc_s3W2 :: Set (ErrorItem (Token [Char])))                                                                                                               
   4   ┊ ┊ ┊ ┊ ┊ ┊(sc_s3W3 :: Set (ErrorItem (Token [Char]))).                                                                                                              
   5   ┊ ┊ ┊ $weerr_Xu (TrivialError                                                                                                                                        
   6   ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊@ [Char]                                                                                                                                          
   7   ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊@ ()                                                                                                                                              
   8   ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊(I# sc_s3W0)                                                                                                                                      
   9   ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊lvl_s2Zq                                                                                                                                          
  10   ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊(Bin                                                                                                                                              
  11   ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ ┊ @ (ErrorItem (Token [Char])) sc_s3W1 lvl_s3l6 sc_s3W2 sc_s3W3))                                                                                
  12   ┊ ┊ ┊ = jump $s$weerr_s3W9 sc_s3W0 sc_s3W1 sc_s3W2 sc_s3W3