QglTF generates shaders with varying names that cause duplicate definitions when loaded with glTFLoader. The loader renames attributes to be automatically usable, so varying or uniform names cannot be the same in the generated shaders.
Forbidden names are:

• position
• normal
• uv
• normalMatrix
• modelViewMatrix
• projectionMatrix
• skinWeight
• skinIndex

For varying/uniform, add something to the name (like normal -> vNormal).