diff --git a/src/librustdoc/clean/cfg.rs b/src/librustdoc/clean/cfg.rs
index 078948cc63bd2633822deaf6dee0814dd8d0546e..fb2cce3683132d720f53cb3d78ceeae8025d55ed 100644
--- a/src/librustdoc/clean/cfg.rs
+++ b/src/librustdoc/clean/cfg.rs
@@ -209,6 +209,9 @@ fn not(self) -> Cfg {
 
 impl ops::BitAndAssign for Cfg {
     fn bitand_assign(&mut self, other: Cfg) {
+        if *self == other {
+            return;
+        }
         match (self, other) {
             (&mut Cfg::False, _) | (_, Cfg::True) => {},
             (s, Cfg::False) => *s = Cfg::False,
@@ -238,6 +241,9 @@ fn bitand(mut self, other: Cfg) -> Cfg {
 
 impl ops::BitOrAssign for Cfg {
     fn bitor_assign(&mut self, other: Cfg) {
+        if *self == other {
+            return;
+        }
         match (self, other) {
             (&mut Cfg::True, _) | (_, Cfg::False) => {},
             (s, Cfg::True) => *s = Cfg::True,
diff --git a/src/test/rustdoc/duplicate-cfg.rs b/src/test/rustdoc/duplicate-cfg.rs
new file mode 100644
index 0000000000000000000000000000000000000000..505d6ee769ab3040c364f7f6830cac26c405f7ac
--- /dev/null
+++ b/src/test/rustdoc/duplicate-cfg.rs
@@ -0,0 +1,15 @@
+#![crate_name = "foo"]
+#![feature(doc_cfg)]
+
+// @has 'foo/index.html'
+// @!has '-' '//*[@class="stab portability"]' 'feature="sync" and'
+// @has '-' '//*[@class="stab portability"]' 'feature="sync"'
+#[doc(cfg(feature = "sync"))]
+#[doc(cfg(feature = "sync"))]
+pub struct Foo;
+
+#[doc(cfg(feature = "sync"))]
+pub mod bar {
+    #[doc(cfg(feature = "sync"))]
+    pub struct Bar;
+}