Pull Request for Issue #30929 .

Summary of Changes

This pull request (PR) adds the necessary SQL statements to delete the duplicate record for plg_sampledata_multilang in the extension table to the update SQL scripts.

I order to not add again a new update SQL script, I've decided to modify the existing script 4.0.0-2020-09-27.sql, which hasn't been released yet and so can be modified, as we don't support updates between nightlies.

If a new Beta or whatever else release of J4 will be made before this PR will be merged, I'll have to move the new statement to a new update SQL script then.

In addition, this PR deletes the update SQL script 4.0.0-2018-06-11.sql which causes the duplicate record. In fact the other one 4.0.0-2019-03-31.sql which was added later with commit fc0d2ea#diff-97d026690fd483a67b57a74953ef4a50 caused the problem, but because no other SQL script in between those references the record for that plugin, it doesn't matter which of the duplicate insert statements is deleted, and so I've chosen the first one because 1. it saves us a complete update SQL script, and 2. the deleted one had the wrong "enabled" status.

Testing Instructions

Test 1: Reproduce the issue by updating a 3.10 to 4.0 Beta 4

1. Update a clean installation (i.e. no 3rd party extensions) of current 3.10-dev or latest 3.10 nightly or 3.10-alpha2 to 4.0 Beta 4, using the following custom URL for Live Update or the following update package for Upload & Update:

• Custom update URL = https://update.joomla.org/core/test/310to4_list.xml
• Update package = https://github.com/joomla/joomla-cms/releases/download/4.0.0-beta4/Joomla_4.0.0-beta4-Beta-Update_Package.zip

2. After successful update, go to the "System - Manage - Plugins" and check if there are plugins with name "Sample Data - Multilingual".
   Result: There are two plugins with that name, one of them is disabled.
3. Go to "System - Manage - Extensions" and search for "Sample Data - Multilingual".
   Result: There are two plugins with that name, one of them is disabled.

Test 2: Verify that this PR fixes the issue when updating a 4.0 Beta 4 which has the issue

This test has to be executed after the previous test "Test 1".

1. Update the installation used in the previous test "Test 1" to current 4.0-dev plus the fix from this PR applied, using the following custom URL for Live Update or the following update package for Upload & Update:

• Custom update URL = https://ci.joomla.org/artifacts/joomla/joomla-cms/4.0-dev/30945/downloads/36230/pr_list.xml
• Update package = https://ci.joomla.org/artifacts/joomla/joomla-cms/4.0-dev/30945/downloads/36230/Joomla_4.0.0-beta5-dev+pr.30945-Development-Update_Package.zip

2. After successful update, go to the "System - Manage - Plugins" and check if there are plugins with name "Sample Data - Multilingual".

Result: There is only one plugin with name "Sample Data - Multilingual", which is enabled. The first one of the two duplicate plugins has been deleted.

Test 3: Verify that this PR fixes the issue when updating from 3.10-dev

Repeat test "Test 1", i.e. start again from 3.10, but this time use the update package of latest 4.0 nightly plus the fix of this PR included. The custom update URL and the download package are the same as used in the previous test "Test 2".

Result: There is only one plugin with name "Sample Data - Multilingual", which is enabled.

Actual result BEFORE applying this Pull Request

After an update from 3.10 to 4, there are two plugins with name "Sample Data - Multilingual". They are equal, except one of them is disabled.

Expected result AFTER applying this Pull Request

After an update from 3.10 to 4, there is only one plugin with name "Sample Data - Multilingual", which is enabled.

After update from a 4 Beta 4 which had the issue to a future 4 version which contains the fix from this PR, one of the duplicate plugins "Sample Data - Multilingual" will be deleted.

Documentation Changes Required

None.