diff --git a/properties.go b/properties.go
index 62ae2d6..f001b03 100644
--- a/properties.go
+++ b/properties.go
@@ -700,22 +700,17 @@ func (p *Properties) Delete(key string) {
 
 // Merge merges properties, comments and keys from other *Properties into p
 func (p *Properties) Merge(other *Properties) {
+	for _, k := range other.k {
+		if _, ok := p.m[k]; !ok {
+			p.k = append(p.k, k)
+		}
+	}
 	for k, v := range other.m {
 		p.m[k] = v
 	}
 	for k, v := range other.c {
 		p.c[k] = v
 	}
-
-outer:
-	for _, otherKey := range other.k {
-		for _, key := range p.k {
-			if otherKey == key {
-				continue outer
-			}
-		}
-		p.k = append(p.k, otherKey)
-	}
 }
 
 // ----------------------------------------------------------------------------
diff --git a/properties_test.go b/properties_test.go
index 33f516c..55d4162 100644
--- a/properties_test.go
+++ b/properties_test.go
@@ -913,6 +913,37 @@ func TestLoad(t *testing.T) {
 
 // ----------------------------------------------------------------------------
 
+var inputs = []struct {
+	input int
+}{
+	{input: 1e2},
+	{input: 1e3},
+	{input: 1e4},
+	{input: 1e5},
+}
+
+func BenchmarkMerge(b *testing.B) {
+	for _, v := range inputs {
+		p := generateProperties(v.input)
+		b.Run(fmt.Sprintf("num_properties_%d", v.input), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				p.Merge(p)
+			}
+		})
+	}
+}
+
+func generateProperties(n int) *Properties {
+	p := NewProperties()
+	for i := 0; i < n; i++ {
+		s := fmt.Sprintf("%v", i)
+		p.Set(s, s)
+	}
+	return p
+}
+
+// ----------------------------------------------------------------------------
+
 // tests all combinations of delimiters, leading and/or trailing whitespace and newlines.
 func testWhitespaceAndDelimiterCombinations(t *testing.T, key, value string) {
 	whitespace := []string{"", " ", "\f", "\t"}