diff --git a/src/SIL.Machine.Morphology.HermitCrab/MorphologicalRules/AnalysisMorphologicalTransform.cs b/src/SIL.Machine.Morphology.HermitCrab/MorphologicalRules/AnalysisMorphologicalTransform.cs
index ed77b3cb3..d0807e32d 100644
--- a/src/SIL.Machine.Morphology.HermitCrab/MorphologicalRules/AnalysisMorphologicalTransform.cs
+++ b/src/SIL.Machine.Morphology.HermitCrab/MorphologicalRules/AnalysisMorphologicalTransform.cs
@@ -90,7 +90,14 @@ out List<List<ShapeNode>> copies
             {
                 GroupCapture<ShapeNode> capture = match.GroupCaptures[GetGroupName(partName, i)];
                 if (!capture.Success)
-                    return false;
+                {
+                    // an empty capture is a zero-length copy, which cannot equal a non-empty one; but a restored
+                    // deletion (an optional node) might fill it, so such a word stays undecidable
+                    if (match.Input.Shape.Any(node => node.Annotation.Optional))
+                        return false;
+                    copies.Add(new List<ShapeNode>());
+                    continue;
+                }
 
                 // an unapplied word-initial deletion sits before the capture, not inside it
                 List<ShapeNode> nodes = MorphologicalOutputAction
diff --git a/tests/SIL.Machine.Morphology.HermitCrab.Tests/MorphologicalRules/PruneDisagreeingCopiesTests.cs b/tests/SIL.Machine.Morphology.HermitCrab.Tests/MorphologicalRules/PruneDisagreeingCopiesTests.cs
index 07314040a..eff3576d7 100644
--- a/tests/SIL.Machine.Morphology.HermitCrab.Tests/MorphologicalRules/PruneDisagreeingCopiesTests.cs
+++ b/tests/SIL.Machine.Morphology.HermitCrab.Tests/MorphologicalRules/PruneDisagreeingCopiesTests.cs
@@ -154,16 +154,44 @@ public void WordInitialOptionalNonSegmentIsNeverPruned()
     }
 
     [Test]
-    public void EmptyCopyIsNeverPruned()
+    public void EmptyCopyAgainstANonEmptyCopyIsPruned()
     {
         (string first, string second) = FindIncompatibleSegments();
         Pattern<Word, ShapeNode> part = Pattern<Word, ShapeNode>.New("copy").Annotation(AnySegment).ZeroOrMore.Value;
         AffixProcessRule rule = CreateCopyRule(part);
         Word input = CreateInput(first + second);
 
-        // the split with an empty copy has nothing to compare, so only the split of two incompatible copies is removed
+        // "whole word + empty" and "empty + whole word" cannot both be copies of one input, and neither can the
+        // split of two incompatible halves
         Assert.That(AnalysisForms(false, rule, input), Is.EquivalentTo(new[] { first, first + second }));
-        Assert.That(AnalysisForms(true, rule, input), Is.EqualTo(new[] { first + second }));
+        Assert.That(AnalysisForms(true, rule, input), Is.Empty);
+    }
+
+    [Test]
+    public void EmptyCopyIsKeptWhenTheWordHasAnOptionalNode()
+    {
+        // a restored deletion could fill the empty copy, so the match stays undecidable
+        (string first, string second) = FindIncompatibleSegments();
+        Pattern<Word, ShapeNode> part = Pattern<Word, ShapeNode>.New("copy").Annotation(AnySegment).ZeroOrMore.Value;
+        AffixProcessRule rule = CreateCopyRule(part);
+        Word input = CreateInputWithOptionalSecondSegment(first + second);
+
+        Assert.That(AnalysisForms(true, rule, input), Is.EquivalentTo(AnalysisForms(false, rule, input)));
+    }
+
+    [Test]
+    public void ZeroOrMoreCopyRuleAddsNoSynthesisWorkForAWordItDoesNotReduplicate()
+    {
+        // sillsdev/machine#634: the match "whole word + empty copy" used to re-run lookup and synthesis
+        string[] words = { "pit", "pʰit", "pʰut", "sagd", "sagdv" };
+        (int lookups, int syntheses) without = CountSearchWork(words);
+
+        Morphophonemic.MorphologicalRules.Add(
+            CreateCopyRule(Pattern<Word, ShapeNode>.New("copy").Annotation(AnySegment).ZeroOrMore.Value)
+        );
+
+        Assert.That(without.syntheses, Is.GreaterThan(0));
+        Assert.That(CountSearchWork(words), Is.EqualTo(without));
     }
 
     [TestCase(false, 3)]
@@ -550,6 +578,32 @@ private RewriteRule CreateBetweenVowelsRewrite(CharacterDefinitionTable table, s
         return rule;
     }
 
+    private (int Lookups, int Syntheses) CountSearchWork(string[] words)
+    {
+        var morpher = new Morpher(new TraceManager { IsTracing = true, RetainWords = false }, Language, 1);
+        int lookups = 0;
+        int syntheses = 0;
+        foreach (string word in words)
+        {
+            morpher.ParseWord(word, out object trace);
+            foreach (Trace node in Flatten((Trace)trace))
+            {
+                if (node.Type == TraceType.LexicalLookup)
+                    lookups++;
+                else if (node.Type == TraceType.WordSynthesis)
+                    syntheses++;
+            }
+        }
+        return (lookups, syntheses);
+    }
+
+    private static IEnumerable<Trace> Flatten(Trace trace)
+    {
+        yield return trace;
+        foreach (Trace descendant in trace.Children.SelectMany(Flatten))
+            yield return descendant;
+    }
+
     private Morpher CreateMorpher(bool pruneCopies, int maxDegreeOfParallelism = 0)
     {
         return new Morpher(TraceManager, Language, maxDegreeOfParallelism) { PruneDisagreeingCopies = pruneCopies };