diff --git a/web/app/brep/operations/boolean.js b/web/app/brep/operations/boolean.js
index 20a64d77..74186939 100644
--- a/web/app/brep/operations/boolean.js
+++ b/web/app/brep/operations/boolean.js
@@ -79,7 +79,9 @@ export function BooleanAlgorithm( shellA, shellB, type ) {
     type = TYPE.INTERSECT;
   }
 
-  let workingFaces = [];
+  let workingFaces = collectFaces(shellA, shellB);
+
+  initOperationData(workingFaces);
 
   mergeVertices(shellA, shellB);
   initVertexFactory(shellA, shellB);
@@ -87,9 +89,6 @@ export function BooleanAlgorithm( shellA, shellB, type ) {
   intersectEdges(shellA, shellB);
   let mergedFaces = mergeOverlappingFaces(shellA, shellB, type);
 
-  initOperationData(shellA, workingFaces);
-  initOperationData(shellB, workingFaces);
-
   intersectFaces(shellA, shellB, type);
 
   replaceMergedFaces(workingFaces, mergedFaces);
@@ -239,7 +238,7 @@ function detectLoops(surface, graph) {
 }
 
 
-function findOverlappingFaces(shell1, shell2) {
+function findOverlappingFaces(shellA, shellB) {
 
   function overlapsImpl(face1, face2) {
     function pointOnFace(face, pt) {
@@ -264,21 +263,23 @@ function findOverlappingFaces(shell1, shell2) {
 
   let overlapGroups = [];
 
-  for (let face1 of shell1.faces) {
-    for (let face2 of shell2.faces) {
+  for (let faceA of shellA.faces) {
+    for (let faceB of shellB.faces) {
       if (DEBUG.FACE_MERGE) {
         __DEBUG__.Clear();
-        __DEBUG__.AddFace(face1, 0x0000ff);
-        __DEBUG__.AddFace(face2);
+        __DEBUG__.AddFace(faceA, 0x0000ff);
+        __DEBUG__.AddFace(faceB);
       }
-      if (overlaps(face1, face2) ) {
-        let group = overlapGroups.find(g => g[0].has(face1) || g[1].has(face2));
+      if (overlaps(faceA, faceB) ) {
+        let group = overlapGroups.find(g => g[0].has(faceA) || g[1].has(faceB));
         if (!group) {
           group = [new Set(), new Set()];    
           overlapGroups.push(group);
         } 
-        group[0].add(face1);
-        group[1].add(face2);
+        group[A].add(faceA);
+        group[B].add(faceB);
+        faceA.op.overlaps = group[B];
+        faceB.op.overlaps = group[A];
       }
     }
   }
@@ -341,12 +342,18 @@ function mergeFaces(facesA, facesB, opType) {
         if (edgeA.vertexA === edgeB.vertexA) {
           // chooseBetweenEqualEdges();
           // canEdgeBeTransferred(edge, face, opType)
-          throw new CadError({
-            kind: CadError.KIND.UNSUPPORTED_CASE,
-            code: 'edge collision on face merge',
-            relatedTopoObjects: [edgeA, edgeB],
-            userMessage: "edges can't be coincident for this operation"
-          });
+          let faceAAdjacent = edgeA.twin().loop.face;
+          let faceBAdjacent = edgeB.twin().loop.face;
+          if (faceAAdjacent.op.overlaps && faceAAdjacent.op.overlaps.has(faceBAdjacent)) {
+            invalid.add(edgeB);
+          } else {
+            throw new CadError({
+              kind: CadError.KIND.UNSUPPORTED_CASE,
+              code: 'edge collision on face merge',
+              relatedTopoObjects: [edgeA, edgeB],
+              userMessage: "edges can't be coincident for this operation"
+            });
+          }
         } else if (edgeA.vertexA === edgeB.vertexB) {
 
           invalid.add(edgeA);
@@ -369,12 +376,12 @@ function mergeFaces(facesA, facesB, opType) {
       }
     }
     
-    function invalidateEdges(faceA, faceB) {
-      for (let edgeA of faceA.edges) {
-        if (coincidentEdges.has(edgeA)) {
+    function invalidateEdges(faceX, faceY) {
+      for (let edgeX of faceX.edges) {
+        if (coincidentEdges.has(edgeX)) {
           continue;
         }
-        invalidateEdge(faceB, edgeA); 
+        invalidateEdge(faceY, edgeX); 
       }
     }
     
@@ -391,7 +398,6 @@ function mergeFaces(facesA, facesB, opType) {
   for (let faceA of facesA) {
     for (let faceB of facesB) {
       invalidate(faceA, faceB);
-      invalidate(faceB, faceA);
     }
   }
 
@@ -522,10 +528,21 @@ export function loopsToFaces(originFace, loops, out) {
   }
 }
 
-function initOperationData(shell, faceCollector) {
-  for (let face of shell.faces) {
+function collectFaces(shellA, shellB) {
+  function collect(shell, out) {
+    for (let face of shell.faces) {
+      out.push(face);
+    }
+  }
+  let out = [];
+  collect(shellA, out);
+  collect(shellB, out);
+  return out;
+}
+
+function initOperationData(faces) {
+  for (let face of faces) {
     initOperationDataForFace(face);
-    faceCollector.push(face);
   }
 }
 
@@ -1160,6 +1177,7 @@ class FaceOperationData extends EdgeGraph {
     this.face = face;
     this.newEdges = [];
     this.collidedEdges = [];
+    this.overlaps = null;
   }
 
   markTransferredFrom(edge) {
diff --git a/web/test/cases/brep-bool-topo.js b/web/test/cases/brep-bool-topo.js
index 4b354d82..1a567ddb 100644
--- a/web/test/cases/brep-bool-topo.js
+++ b/web/test/cases/brep-bool-topo.js
@@ -261,6 +261,22 @@ export default defineTests([
       'vertices': [[-250, -250, -250], [-250, -250, -163], [-250, -250, 157], [-250, -250, 250], [-250, 250, -250], [-250, 250, -163], [-250, 250, 157], [-250, 250, 250], [250, -250, -250], [250, -250, -163], [250, -250, 157], [250, -250, 250], [250, 250, -250], [250, 250, -163], [250, 250, 157], [250, 250, 250]],
       'faces': [[[7, 3, 11, 15]], [[8, 0, 4, 12]], [[5, 1, 9, 13]], [[2, 6, 14, 10]], [[2, 10, 11, 3]], [[9, 1, 0, 8]], [[13, 9, 8, 12]], [[10, 14, 15, 11]], [[5, 13, 12, 4]], [[14, 6, 7, 15]], [[6, 2, 3, 7]], [[1, 5, 4, 0]]]
     }
+  },
+
+  {
+    'name': '4AdjacentFacesUnion_BUG_47',
+    'state': {
+      'sketches': {'0:3': {'Segment': [[[-379, 250], [157, 250]], [[157, 250], [157, -250]], [[157, -250], [-379, -250]], [[-379, -250], [-379, 250]]]}},
+      'operations': [{'type': 'BOX', 'params': {'width': 500, 'height': 500, 'depth': 500}}, {
+        'type': 'EXTRUDE',
+        'params': {'value': -500, 'prism': 1, 'angle': 0, 'rotation': 0, 'face': '0:3'}
+      }]
+    },
+    'expected': {
+      'format': 'LOOPS',
+      'vertices': [[-250, -250, -379], [-250, -250, -250], [-250, -250, 157], [-250, -250, 250], [-250, 250, -379], [-250, 250, -250], [-250, 250, 157], [-250, 250, 250], [250, -250, -379], [250, -250, -250], [250, -250, 157], [250, -250, 250], [250, 250, -379], [250, 250, -250], [250, 250, 157], [250, 250, 250]],
+      'faces': [[[7, 3, 11, 15]], [[8, 0, 4, 12]], [[0, 8, 9, 10, 11, 3, 2, 1]], [[9, 8, 12, 13, 14, 15, 11, 10]], [[13, 12, 4, 5, 6, 7, 15, 14]], [[6, 5, 4, 0, 1, 2, 3, 7]]]
+    }
   }
 
 ]);