Same points converted twice: offset grows from 555 m to 1110 m

• Beijing Guomao 116.397428, 39.909230 should become 116.403672, 39.910634 (555.6 m) but the map shows 116.409922, 39.912036, about 1110 m away.
• The error direction stays north-east, so it looks like a larger datum offset rather than a reversed conversion.
• The whole POI layer moves by the same doubled distance, which no longer looks like survey error.

1. Take one known point, subtract the interface value from the on-map value to get the actual offset vector.
2. Compute the single-pass WGS84 to GCJ-02 result and compare it with the API return; a match means the backend already converted.
3. Copy the API response into gcoord.transform and run it once in the console to see whether it moves another ~555 m.
4. Search the frontend for a global coordinate middleware or an axios response interceptor doing the conversion.
5. Check whether the stored rows carry any CRS marker at all; if not, the pipeline is guessing.

Convert exactly once, preferably at the data-production end, and write the CRS into the payload so downstream code never has to infer it.

const a = turf.point([116.397428, 39.909230]);   // WGS84 原始点
const b = turf.point([116.403672, 39.910634]);   // 应得 GCJ-02
const c = turf.point([116.409922, 39.912036]);   // 实际渲染点

turf.distance(a, b, { units: 'meters' });  // 555.6
turf.distance(a, c, { units: 'meters' });  // 1110.2
turf.distance(b, c, { units: 'meters' });  // 554.6

{
  "type": "FeatureCollection",
  "coordSys": "GCJ02",
  "features": [
    { "type": "Feature",
      "properties": { "name": "国贸" },
      "geometry": { "type": "Point", "coordinates": [116.403672, 39.910634] } }
  ]
}
const SRC = 'WGS84', DST = 'GCJ02';
function normalize(fc, target) {
  const from = fc.coordSys || 'WGS84';
  if (from === target) return fc;                    // 已经一致，直接返回
  return gcoord.transform(fc, gcoord[from], gcoord[target]);
}
const data = normalize(resp.data, 'GCJ02');

• Trying to invert the double conversion numerically: f(f(x)) cannot be undone by subtracting the offset, polluted rows must be re-exported from the original WGS84 source.
• Adding a third conversion to 'fix' the doubled offset, which pushes the layer another 555 m north-east.