diff --git a/Lib/fontTools/feaLib/ast.py b/Lib/fontTools/feaLib/ast.py
index bbe6e6e74..17c6cc3fb 100644
--- a/Lib/fontTools/feaLib/ast.py
+++ b/Lib/fontTools/feaLib/ast.py
@@ -1272,15 +1272,18 @@ class MultipleSubstStatement(Statement):
             replaces.append(replace)
         replaces = list(zip(*replaces))
 
+        seen_originals = set()
         for i, original in enumerate(originals):
-            builder.add_multiple_subst(
-                self.location,
-                prefix,
-                original,
-                suffix,
-                replaces and replaces[i] or (),
-                self.forceChain,
-            )
+            if original not in seen_originals:
+                seen_originals.add(original)
+                builder.add_multiple_subst(
+                    self.location,
+                    prefix,
+                    original,
+                    suffix,
+                    replaces and replaces[i] or (),
+                    self.forceChain,
+                )
 
     def asFea(self, indent=""):
         res = "sub "
diff --git a/Tests/feaLib/data/GSUB_2.fea b/Tests/feaLib/data/GSUB_2.fea
index 078cbec39..21db452b5 100644
--- a/Tests/feaLib/data/GSUB_2.fea
+++ b/Tests/feaLib/data/GSUB_2.fea
@@ -29,3 +29,7 @@ lookup l1 {
 feature f5 {
    sub @class' lookup l1 [i l];
 } f5;
+
+feature f6 {
+    sub [f_i f_i]' j by [f f] [i i];
+} f6;
diff --git a/Tests/feaLib/data/GSUB_2.ttx b/Tests/feaLib/data/GSUB_2.ttx
index b2bd21bd7..fb87a0590 100644
--- a/Tests/feaLib/data/GSUB_2.ttx
+++ b/Tests/feaLib/data/GSUB_2.ttx
@@ -10,19 +10,20 @@
         <Script>
           <DefaultLangSys>
             <ReqFeatureIndex value="65535"/>
-            <!-- FeatureCount=5 -->
+            <!-- FeatureCount=6 -->
             <FeatureIndex index="0" value="0"/>
             <FeatureIndex index="1" value="1"/>
             <FeatureIndex index="2" value="2"/>
             <FeatureIndex index="3" value="3"/>
             <FeatureIndex index="4" value="4"/>
+            <FeatureIndex index="5" value="5"/>
           </DefaultLangSys>
           <!-- LangSysCount=0 -->
         </Script>
       </ScriptRecord>
     </ScriptList>
     <FeatureList>
-      <!-- FeatureCount=5 -->
+      <!-- FeatureCount=6 -->
       <FeatureRecord index="0">
         <FeatureTag value="f1  "/>
         <Feature>
@@ -58,9 +59,16 @@
           <LookupListIndex index="0" value="5"/>
         </Feature>
       </FeatureRecord>
+      <FeatureRecord index="5">
+        <FeatureTag value="f6  "/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="6"/>
+        </Feature>
+      </FeatureRecord>
     </FeatureList>
     <LookupList>
-      <!-- LookupCount=6 -->
+      <!-- LookupCount=8 -->
       <Lookup index="0">
         <LookupType value="2"/>
         <LookupFlag value="0"/>
@@ -135,6 +143,35 @@
           </SubstLookupRecord>
         </ChainContextSubst>
       </Lookup>
+      <Lookup index="6">
+        <LookupType value="6"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ChainContextSubst index="0" Format="3">
+          <!-- BacktrackGlyphCount=0 -->
+          <!-- InputGlyphCount=1 -->
+          <InputCoverage index="0">
+            <Glyph value="f_i"/>
+          </InputCoverage>
+          <!-- LookAheadGlyphCount=1 -->
+          <LookAheadCoverage index="0">
+            <Glyph value="j"/>
+          </LookAheadCoverage>
+          <!-- SubstCount=1 -->
+          <SubstLookupRecord index="0">
+            <SequenceIndex value="0"/>
+            <LookupListIndex value="7"/>
+          </SubstLookupRecord>
+        </ChainContextSubst>
+      </Lookup>
+      <Lookup index="7">
+        <LookupType value="2"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <MultipleSubst index="0">
+          <Substitution in="f_i" out="f,i"/>
+        </MultipleSubst>
+      </Lookup>
     </LookupList>
   </GSUB>