diff --git a/builder/builder-next/worker/worker.go b/builder/builder-next/worker/worker.go
index 271192cc7a..64771973a2 100644
--- a/builder/builder-next/worker/worker.go
+++ b/builder/builder-next/worker/worker.go
@@ -158,15 +158,7 @@ func (w *Worker) Labels() map[string]string {
 // Platforms returns one or more platforms supported by the image.
 func (w *Worker) Platforms(noCache bool) []ocispec.Platform {
 	if noCache {
-		pm := make(map[string]struct{}, len(w.Opt.Platforms))
-		for _, p := range w.Opt.Platforms {
-			pm[platforms.Format(p)] = struct{}{}
-		}
-		for _, p := range archutil.SupportedPlatforms(noCache) {
-			if _, ok := pm[platforms.Format(p)]; !ok {
-				w.Opt.Platforms = append(w.Opt.Platforms, p)
-			}
-		}
+		w.Opt.Platforms = mergePlatforms(w.Opt.Platforms, archutil.SupportedPlatforms(noCache))
 	}
 	if len(w.Opt.Platforms) == 0 {
 		return []ocispec.Platform{platforms.DefaultSpec()}
@@ -174,6 +166,30 @@ func (w *Worker) Platforms(noCache bool) []ocispec.Platform {
 	return w.Opt.Platforms
 }
 
+// mergePlatforms merges the defined platforms with the supported platforms
+// and returns a new slice of platforms. It ensures no duplicates.
+func mergePlatforms(defined, supported []ocispec.Platform) []ocispec.Platform {
+	result := []ocispec.Platform{}
+	matchers := make([]platforms.MatchComparer, len(defined))
+	for i, p := range defined {
+		result = append(result, p)
+		matchers[i] = platforms.Only(p)
+	}
+	for _, p := range supported {
+		exists := false
+		for _, m := range matchers {
+			if m.Match(p) {
+				exists = true
+				break
+			}
+		}
+		if !exists {
+			result = append(result, p)
+		}
+	}
+	return result
+}
+
 // GCPolicy returns automatic GC Policy
 func (w *Worker) GCPolicy() []client.PruneInfo {
 	return w.Opt.GCPolicy
diff --git a/builder/builder-next/worker/worker_test.go b/builder/builder-next/worker/worker_test.go
new file mode 100644
index 0000000000..f076068c1f
--- /dev/null
+++ b/builder/builder-next/worker/worker_test.go
@@ -0,0 +1,76 @@
+package worker
+
+import (
+	"testing"
+
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"gotest.tools/v3/assert"
+	is "gotest.tools/v3/assert/cmp"
+)
+
+func TestMergePlatforms(t *testing.T) {
+	defaultPlatform := ocispec.Platform{OS: "linux", Architecture: "amd64"}
+	otherPlatform := ocispec.Platform{OS: "windows", Architecture: "amd64"}
+	thirdPlatform := ocispec.Platform{OS: "darwin", Architecture: "arm64"}
+
+	tests := []struct {
+		name      string
+		defined   []ocispec.Platform
+		supported []ocispec.Platform
+		contains  []ocispec.Platform
+		wantLen   int
+	}{
+		{
+			name:      "AllUnique",
+			defined:   []ocispec.Platform{defaultPlatform},
+			supported: []ocispec.Platform{otherPlatform, thirdPlatform},
+			contains:  []ocispec.Platform{defaultPlatform, otherPlatform, thirdPlatform},
+			wantLen:   3,
+		},
+		{
+			name:      "SomeOverlap",
+			defined:   []ocispec.Platform{defaultPlatform, otherPlatform},
+			supported: []ocispec.Platform{otherPlatform, thirdPlatform},
+			contains:  []ocispec.Platform{defaultPlatform, otherPlatform, thirdPlatform},
+			wantLen:   3,
+		},
+		{
+			name:      "AllOverlap",
+			defined:   []ocispec.Platform{defaultPlatform, otherPlatform},
+			supported: []ocispec.Platform{defaultPlatform, otherPlatform},
+			contains:  []ocispec.Platform{defaultPlatform, otherPlatform},
+			wantLen:   2,
+		},
+		{
+			name:      "EmptySupported",
+			defined:   []ocispec.Platform{defaultPlatform},
+			supported: []ocispec.Platform{},
+			contains:  []ocispec.Platform{defaultPlatform},
+			wantLen:   1,
+		},
+		{
+			name:      "EmptyDefined",
+			defined:   []ocispec.Platform{},
+			supported: []ocispec.Platform{defaultPlatform, otherPlatform},
+			contains:  []ocispec.Platform{defaultPlatform, otherPlatform},
+			wantLen:   2,
+		},
+		{
+			name:      "BothEmpty",
+			defined:   []ocispec.Platform{},
+			supported: []ocispec.Platform{},
+			contains:  []ocispec.Platform{},
+			wantLen:   0,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := mergePlatforms(tc.defined, tc.supported)
+			assert.Equal(t, len(got), tc.wantLen)
+			for _, p := range tc.contains {
+				assert.Assert(t, is.Contains(got, p))
+			}
+		})
+	}
+}