Interestingly, using instead the rawcontinue does not give you a boolean (as described in the API help near the bottom), but the next gaicontinue filename

https://commons.wikimedia.org/w/api.php?action=query&generator=allimages&prop=duplicatefiles&gaifrom=3D%F0%AA%8E%A5-seal.svg&gailimit=1&format=json&rawcontinue=

{

  "query-continue": {
    "allimages": {
      "gaicontinue": "3E-100.jpg"
    }
  },
  "query": {
    "pages": {
      "4236541": {
        "pageid": 4236541,
        "ns": 6,
        "title": "File:3E-003 PTKiGK Zabrze Naklo Slaskie 18062008 01.jpg"
      }
    }
  }
}