Only some duplicates are being picked up

**Describe the bug**

Not all word duplicates are being picked up. It looks like there is a list of common duplicates which gets checked for, rather than looking for everything.

**To Reproduce**
Steps to reproduce the behavior:

1. Enter the text below
2. Note that there are a number of duplicates, but Harper only flags up some of them (one, duplicates, picked and problem are all duplicated but not flagged)

```
* There there is always at least one one way to resolve a problem problem.
* Sometimes duplicates duplicates aren't being picked picked up.
* It looks like the the common duplicates only only are being picked up up.
```

**Expected behavior**

All duplicate words should be given an error

**Screenshots**

**Platform**

VSCode.

Start by reproducing the report in VSCode with the three sample sentences and inspect the Rust duplicate-word detection path. Compare the words that are flagged with those that are missed, then add regression coverage for the examples so that every repeated word is reported.

Written by the indexing model from the issue text.