@Peter is correct. If you want to read again and again, you would probably want to read as stream and seek to beginning every time you read the stream. But then if you want to do what do you now but get the second read working, you can seek to the beginning of the stream, after the first read, like this.
await httpContent.LoadIntoBufferAsync();
var X = await httpContent.ReadAsAsync<T>();
Stream stream = await httpContent.ReadAsStreamAsync();
stream.Seek(0, SeekOrigin.Begin);
var Y = await httpContent.ReadAsAsync<Dictionary<string, object>>();