diff --git a/src/cmd/go/internal/modfetch/fetch.go b/src/cmd/go/internal/modfetch/fetch.go
index a7c8c2c769..426df9bc04 100644
--- a/src/cmd/go/internal/modfetch/fetch.go
+++ b/src/cmd/go/internal/modfetch/fetch.go
@@ -833,6 +833,7 @@ Outer:
 		for _, m := range mods {
 			list := goSum.m[m]
 			sort.Strings(list)
+			str.Uniq(&list)
 			for _, h := range list {
 				st := goSum.status[modSum{m, h}]
 				if (!st.dirty || (st.used && keep[m])) && !sumInWorkspaceModulesLocked(m) {
diff --git a/src/cmd/go/testdata/script/mod_tidy_duplicates.txt b/src/cmd/go/testdata/script/mod_tidy_duplicates.txt
new file mode 100644
index 0000000000..d454c8dc82
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_duplicates.txt
@@ -0,0 +1,38 @@
+env GO111MODULE=on
+
+# Regression test for golang.org/issue/28456:
+# 'go mod tidy' should not leave duplicate lines when re-writing the file.
+
+go mod tidy
+cmp go.sum golden.sum
+
+-- go.mod --
+module use
+
+go 1.16
+
+require rsc.io/quote v1.5.2
+
+-- go.sum --
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=
+-- golden.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=
+-- main.go --
+package use
+
+import _ "rsc.io/quote"