Cppcheck may be invoked multiple times while on the fly inspection is in progress

### Environment

* Operating System (e.g. Ubuntu 16.04 x64): Windows 10 Version 1909
* IDE Version (e.g. CLion 2016.3.2): Clion 2020.1 (201.6668.86)
* Cppcheck executable version (`cppcheck --version`): 1.90
* Cppcheck plugin version: 1.4.2

### Expected behaviour

Cppcheck is run only once per file in case it is not modified.

### Actual behaviour

Cppcheck is invoked every time the IDE regains focus.

### Steps to reproduce the behaviour

I have a file which takes extremely long to perform the on the fly inspections on (probably some issue causing this) and while the file is still being being running the inspections Cppcheck will be invoked every time the IDE regains focus (unless it is already running). This does not occur when the inspections are complete.

An example would be `valueflow.h` from https://github.com/danmar/cppcheck.