fix(entities): correct behavior of upsertEntities with duplicate entities

[Char2sGu]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/elf/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #518

What is the new behavior?

For duplicate entities, the latter one will be merged into the former one. The resulting ids array will not have duplicate ids.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.