cache with concurrent requests

I have a page with big slider and thumbs under it. I put resized images in both, but source images are the same.
Source images goes from s3. And thumbs are not loading with error. In verbose mode I see following:

31.07.2019 16:52:342019/07/31 13:52:34 request: {Method:GET URL:https://someminio.com/dev/5b067fd85d773_799bad5a3b514f096e69bbc4a7896cd9.jpg#200x200,fit Proto:HTTP/1.1 ProtoMajor:1 ProtoMinor:1 Header:map[Accept:[image/*] User-Agent:[willnorris/imageproxy]] Body:<nil> GetBody:<nil> ContentLength:0 TransferEncoding:[] Close:false Host:someminio.com Form:map[] PostForm:map[] MultipartForm:<nil> Trailer:map[] RemoteAddr: RequestURI: TLS:<nil> Cancel:<nil> Response:<nil> ctx:<nil>} (served from cache: true)
31.07.2019 16:52:342019/07/31 13:52:34 content-type not allowed: "text/html"

If I turn cache off everything is ok. I assume that second request arrives when cache is in progress and somehow damage it

Start by reproducing the failure with caching enabled and concurrent requests for the same S3 image, using the verbose request output as a reference. Trace the cache path that reports the response as served from cache and compare it with a request made with caching disabled. Done means concurrent thumbnail requests return the image successfully without the text/html content-type error.