RIS import takes the wrong date and duplicates abstract

[Gena928]

Fixes #6065

1. Fixed an issue with "duplicate values in 'Abstract field';
2. Checked code and couldn't reproduce second error: "publication date is taken from the Y2 field, not from the correct Y1"
   See lines 207-220 of RisImporter.java. It works fine and importing publication date as expected. Tested on file from @crystalfp (see file, attached to issue).

Kind regards,
Gennadiy

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[Siedlerchr]

Thanks for contributing looks good to me!

[tobiasdiez]

The change looks good to. Thanks! Could you please add a test that makes sure the example provided in #6065 is now indeed properly imported.
For this create the ris file and the corresponding bib file here: https://github.com/JabRef/jabref/tree/master/src/test/resources/org/jabref/logic/importer/fileformat (they are then automatically picked up).

[Gena928]

OK, will do.

[tobiasdiez]

Thanks!