make unit test TestXPathMapperInMemory.testGetNormalizedPreimageSelector() work

The test fails due to duplicate values:

```
e.wwu.scdh.annotation.selection.mapping.LookupCreationException: java.lang.IllegalArgumentException: value already present: /TEI[1]/text[1]/body[1]/lg[1]/l[7]/text()[1]
```

**Assignees:** @lueck

*Migrated from GitLab: https://zivgitlab.uni-muenster.de/SCDH/selectors/selection-engine/-/issues/11*