Always allow duplicates with identical CRC32 and size

Don't warn on duplicate files in merge_zips if they have identical
CRC32 and size values.

Test: m checkbuild
Test: merge_zips_test.go
Change-Id: I61336ca4d4d3b7402c24a7abd337bd350fe10930

cmd/merge_zips/merge_zips_test.go

@@ -87,6 +87,14 @@ func TestMergeZips(t *testing.T) {
 
 			ignoreDuplicates: true,
 		},
+		{
+			name: "duplicates identical",
+			in: [][]testZipEntry{
+				{a},
+				{a},
+			},
+			out: []testZipEntry{a},
+		},
 		{
 			name: "sort",
 			in: [][]testZipEntry{