Using multiple files with same htdevtools instance

I am currently trying:

```
hdevtools check file1.hs
hdevtools check file2.hs
hdevtools check file1.hs
```

and realized that all benefits of hdevtools are lost: Checking `file1.hs` the second time creates a lot of CPU usage, not using any cached results.

Why is that?

It is also interesting that despite losing the hdevtools benefits, hdevtools still uses a lot of RAM: Loading a few hundred files easily eats up 10 GB RAM. So there seems to be some caching going on, but not actually being used.

How am I supposed to use hdevtools with multiple files?