<AWT Dev> RFR: 8257438: Avoid adding duplicate values into extendedKeyCodesSet

[Turbanov Andrey]

Looks like extended key codes set was filled with _synonyms_
https://wiki.linuxquestions.org/wiki/List_of_Keysyms_Recognised_by_Xmodmap
`0x10006cc` is a _Farsi_yeh_ and also deprecated synonym _Arabic_farsi_yeh_
`0x1000589` is an _Armenian_full_stop_ and also deprecated synonym _Armenian_verjaket_
`0x100055d` is an _Armenian_separation_mark_ and also deprecated synonym _Armenian_but_
`0x100055b` is an _Armenian_accent_ and also deprecated synonym _Armenian_shesht_
`0x100055e` is an _Armenian_question_ and also deprecated synonym _Armenian_paruyk_

Found by IntelliJ IDEA inspection `Java | Probable bugs | Overwritten Map key or Set element`

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Commit messages:
 - [PATCH] Avoid adding duplicate values into extendedKeyCodesSet

Changes: https://git.openjdk.java.net/jdk/pull/1496/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1496&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257438
  Stats: 7 lines in 1 file changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1496.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1496/head:pull/1496

PR: https://git.openjdk.java.net/jdk/pull/1496