diff --git a/src/merlin.ml b/src/merlin.ml
index 5e2bbc62d1fe..a5d0cb233974 100644
--- a/src/merlin.ml
+++ b/src/merlin.ml
@@ -14,20 +14,19 @@ module Preprocess = struct
     | _, (Action _ as action) -> action
     | (Future_syntax _ as future_syntax), _
     | _, (Future_syntax _ as future_syntax) -> future_syntax
-    | Pps { loc = _; pps = pps1; flags = flags1; staged = s1 },
+    | Pps { loc ; pps = pps1; flags = flags1; staged = s1 },
       Pps { loc = _; pps = pps2; flags = flags2; staged = s2 } ->
-      match
-        match Bool.compare s1 s2 with
-        | Gt| Lt as ne -> ne
-        | Eq ->
-          match List.compare flags1 flags2 ~compare:String.compare with
-          | Gt | Lt as ne -> ne
-          | Eq ->
-            List.compare pps1 pps2 ~compare:(fun (_, a) (_, b) ->
-              Lib_name.compare a b)
-      with
-      | Eq -> a
-      | _  -> No_preprocessing
+      if Bool.(<>) s1 s2
+      || List.compare flags1 flags2 ~compare:String.compare <> Eq then
+        No_preprocessing
+      else
+        let pps =
+          let not_in_pps1 (_, pp) =
+            List.for_all pps1 ~f:(fun (_, pp') ->
+              Lib_name.compare pp' pp <> Eq) in
+          pps1 @ List.filter ~f:not_in_pps1 pps2
+        in
+        Pps { loc ; pps; flags = flags1 ; staged = s1 }
 end
 
 let quote_for_merlin s =
diff --git a/test/blackbox-tests/test-cases/github1946/run.t b/test/blackbox-tests/test-cases/github1946/run.t
index 6a18e6deefcf..7477c110f256 100644
--- a/test/blackbox-tests/test-cases/github1946/run.t
+++ b/test/blackbox-tests/test-cases/github1946/run.t
@@ -1,2 +1,2 @@
   $ dune build --display short --profile release | grep "FLG -ppx"
-  [1]
+  FLG -ppx '$TESTCASE_ROOT/_build/default/.ppx/4e93360991c22787c855ecab6b44c222/ppx.exe --as-ppx --cookie '\''library-name="usesppx1"'\'''