Clone an already-loaded FBX model in three.js and be able to animate it. Adapted from: https://gist.github.com/cdata/f2d7a6ccdec071839bc1954c32595e87

cloneFbx.js

const cloneFbx = (fbx) => {
    const clone = fbx.clone(true)
    clone.animations = fbx.animations
    clone.skeleton = { bones: [] }

    const skinnedMeshes = {}

    fbx.traverse(node => {
        if (node.isSkinnedMesh) {
            skinnedMeshes[node.name] = node
        }
    })

    const cloneBones = {}
    const cloneSkinnedMeshes = {}

    clone.traverse(node => {
        if (node.isBone) {
            cloneBones[node.name] = node
        }

        if (node.isSkinnedMesh) {
            cloneSkinnedMeshes[node.name] = node
        }
    })

    for (let name in skinnedMeshes) {
        const skinnedMesh = skinnedMeshes[name]
        const skeleton = skinnedMesh.skeleton
        const cloneSkinnedMesh = cloneSkinnedMeshes[name]

        const orderedCloneBones = []

        for (let i=0; i<skeleton.bones.length; i++) {
            const cloneBone = cloneBones[skeleton.bones[i].name]
            orderedCloneBones.push(cloneBone)
        }

        cloneSkinnedMesh.bind(
            new THREE.Skeleton(orderedCloneBones, skeleton.boneInverses),
            cloneSkinnedMesh.matrixWorld)

        // For animation to work correctly:
        clone.skeleton.bones.push(cloneSkinnedMesh)
        clone.skeleton.bones.push(...orderedCloneBones)
    }

    return clone
}