fs: fix duplicate files causing spurious copies

Before this fix duplicate files (on Google Drive) caused the next file
to be spuriously copied.  `rclone dedupe` worked around the problem.

fs/march.go

@@ -268,20 +268,22 @@ func matchListings(srcListEntries, dstListEntries DirEntries, transforms []match
 			prev := srcList[iSrc-1].name
 			if srcName == prev {
 				Logf(src, "Duplicate %s found in source - ignoring", DirEntryType(src))
-				src = nil // ignore the src
+				iDst-- // ignore the src and retry the dst
+				continue
 			} else if srcName < prev {
-				Errorf(src, "Out of order listing in source")
-				src = nil // ignore the src
+				// this should never happen since we sort the listings
+				panic("Out of order listing in source")
 			}
 		}
 		if dst != nil && iDst > 0 {
 			prev := dstList[iDst-1].name
 			if dstName == prev {
 				Logf(dst, "Duplicate %s found in destination - ignoring", DirEntryType(dst))
-				dst = nil // ignore the dst
+				iSrc-- // ignore the dst and retry the src
+				continue
 			} else if dstName < prev {
-				Errorf(dst, "Out of order listing in destination")
-				dst = nil // ignore the dst
+				// this should never happen since we sort the listings
+				panic("Out of order listing in destination")
 			}
 		}
 		if src != nil && dst != nil {

fs/march_test.go

@@ -101,11 +101,15 @@ func TestMatchListings(t *testing.T) {
 		{
 			what: "One duplicate",
 			input: DirEntries{
+				A, A,
 				a, a,
 				a, nil,
+				b, b,
 			},
 			matches: []matchPair{
+				{A, A},
 				{a, a},
+				{b, b},
 			},
 		},
 		{
@@ -141,20 +145,6 @@ func TestMatchListings(t *testing.T) {
 			},
 			transforms: []matchTransformFn{strings.ToLower},
 		},
-		/*{
-			what: "Out of order",
-			input: DirEntries{
-				c, nil,
-				b, b,
-				a, nil,
-			},
-			srcOnly: DirEntries{
-				c,
-			},
-			dstOnly: DirEntries{
-				b,
-			},
-		},*/
 	} {
 		var srcList, dstList DirEntries
 		for i := 0; i < len(test.input); i += 2 {