Revert "Revert "cmd/compile: discard duplicate inline method bodies""

This reverts commit f612cd70.

Reason for revert: We thought the original change had broken the
linux/amd64 and linux/386 builders, but it turned out to be a problem
with the build infrastructure, not the change.

Change-Id: Ic3318a63464fcba8d845ac04494115a7ba620364
Reviewed-on: https://go-review.googlesource.com/55050
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/bimport.go

@@ -484,6 +484,7 @@ func (p *importer) typ() *types.Type {
 
 		t = pkgtype(p.imp, tsym)
 		p.typList = append(p.typList, t)
+		dup := !t.IsKind(types.TFORW) // type already imported
 
 		// read underlying type
 		t0 := p.typ()
@@ -514,10 +515,19 @@ func (p *importer) typ() *types.Type {
 			result := p.paramList()
 			nointerface := p.bool()
 
+			mt := functypefield(recv[0], params, result)
+			addmethod(sym, mt, false, nointerface)
+
+			if dup {
+				// An earlier import already declared this type and its methods.
+				// Discard the duplicate method declaration.
+				p.funcList = append(p.funcList, nil)
+				continue
+			}
+
 			n := newfuncname(methodname(sym, recv[0].Type))
-			n.Type = functypefield(recv[0], params, result)
+			n.Type = mt
 			checkwidth(n.Type)
-			addmethod(sym, n.Type, false, nointerface)
 			p.funcList = append(p.funcList, n)
 			importlist = append(importlist, n)