fix #7829

[sokra]

webpack-hot-client seem to call addEntry multiple times
which causes two Entrypoints with the same name
This lead the bad side effects
i. e. optimization.runtimeChunk no longer works correctly

Now adding an entry with the same name replaces the existing entry

What kind of change does this PR introduce?
bugfix

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
nothing

[webpack-bot]

For maintainers only:

• This need to be documented (issue in webpack/webpack.js.org will be filed when merged)

[webpack-bot]

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