diff --git a/src/uniforms/MaterialsTexture.js b/src/uniforms/MaterialsTexture.js
index 4d46c263e..3a9bf9888 100644
--- a/src/uniforms/MaterialsTexture.js
+++ b/src/uniforms/MaterialsTexture.js
@@ -1,4 +1,5 @@
 import { DataTexture, RGBAFormat, ClampToEdgeWrapping, FloatType, FrontSide, BackSide, DoubleSide } from 'three';
+import { reduceTexturesToUniqueSources } from './utils.js';
 
 const MATERIAL_PIXELS = 45;
 const MATERIAL_STRIDE = MATERIAL_PIXELS * 4;
@@ -58,7 +59,16 @@ export class MaterialsTexture extends DataTexture {
 
 		function getTexture( material, key, def = - 1 ) {
 
-			return key in material ? textures.indexOf( material[ key ] ) : def;
+			if ( key in material && material[ key ] ) {
+
+				const source = material[ key ].source;
+				return uniqueTextures.findIndex( tex => tex.source === source );
+
+			} else {
+
+				return def;
+
+			}
 
 		}
 
@@ -140,6 +150,9 @@ export class MaterialsTexture extends DataTexture {
 		const dimension = Math.ceil( Math.sqrt( pixelCount ) );
 		const { threeCompatibilityTransforms, image } = this;
 
+		// get the list of textures with unique sources
+		const uniqueTextures = reduceTexturesToUniqueSources( textures );
+
 		if ( image.width !== dimension ) {
 
 			this.dispose();
@@ -170,9 +183,9 @@ export class MaterialsTexture extends DataTexture {
 			// sample 1
 			// metalness & roughness
 			floatArray[ index ++ ] = getField( m, 'metalness', 0.0 );
-			floatArray[ index ++ ] = textures.indexOf( m.metalnessMap );
+			floatArray[ index ++ ] = uniqueTextures.indexOf( m.metalnessMap );
 			floatArray[ index ++ ] = getField( m, 'roughness', 0.0 );
-			floatArray[ index ++ ] = textures.indexOf( m.roughnessMap );
+			floatArray[ index ++ ] = uniqueTextures.indexOf( m.roughnessMap );
 
 			// sample 2
 			// transmission & emissiveIntensity
diff --git a/src/uniforms/RenderTarget2DArray.js b/src/uniforms/RenderTarget2DArray.js
index 66f7711bf..7dc7efa85 100644
--- a/src/uniforms/RenderTarget2DArray.js
+++ b/src/uniforms/RenderTarget2DArray.js
@@ -9,6 +9,7 @@ import {
 	NoToneMapping,
 } from 'three';
 import { FullScreenQuad } from 'three/examples/jsm/postprocessing/Pass.js';
+import { reduceTexturesToUniqueSources } from './utils.js';
 
 const prevColor = new Color();
 export class RenderTarget2DArray extends WebGLArrayRenderTarget {
@@ -37,6 +38,9 @@ export class RenderTarget2DArray extends WebGLArrayRenderTarget {
 
 	setTextures( renderer, width, height, textures ) {
 
+		// get the list of textures with unique sources
+		const uniqueTextures = reduceTexturesToUniqueSources( textures );
+
 		// save previous renderer state
 		const prevRenderTarget = renderer.getRenderTarget();
 		const prevToneMapping = renderer.toneMapping;
@@ -45,7 +49,7 @@ export class RenderTarget2DArray extends WebGLArrayRenderTarget {
 
 		// resize the render target and ensure we don't have an empty texture
 		// render target depth must be >= 1 to avoid unbound texture error on android devices
-		const depth = textures.length || 1;
+		const depth = uniqueTextures.length || 1;
 		this.setSize( width, height, depth );
 		renderer.setClearColor( 0, 0 );
 		renderer.toneMapping = NoToneMapping;
@@ -54,7 +58,7 @@ export class RenderTarget2DArray extends WebGLArrayRenderTarget {
 		const fsQuad = this.fsQuad;
 		for ( let i = 0, l = depth; i < l; i ++ ) {
 
-			const texture = textures[ i ];
+			const texture = uniqueTextures[ i ];
 			if ( texture ) {
 
 				// revert to default texture transform before rendering
diff --git a/src/uniforms/utils.js b/src/uniforms/utils.js
new file mode 100644
index 000000000..ec378b64c
--- /dev/null
+++ b/src/uniforms/utils.js
@@ -0,0 +1,21 @@
+// reduce the set of textures to just those with a unique source while retaining
+// the order of the textures.
+export function reduceTexturesToUniqueSources( textures ) {
+
+	const sourceSet = new Set();
+	const result = [];
+	for ( let i = 0, l = textures.length; i < l; i ++ ) {
+
+		const tex = textures[ i ];
+		if ( ! sourceSet.has( tex.source ) ) {
+
+			sourceSet.add( tex.source );
+			result.push( tex );
+
+		}
+
+	}
+
+	return result;
+
+}