In 4.20, calling BuildPlugin on a plugin outside the \\Engine\Plugins\... folder whose .uplugin has the same filename as a plugin inside \\Engine\Plugins\... generates the following error:

ERROR: Found 'TestPlugin' plugin in two locations (C:\PluginDir\BuiltPlugin\HostProject\Plugins\TestPlugin\TestPlugin.uplugin and C:\Program Files\Epic Games\UE_4.20\Engine\Plugins\Marketplace\TestPlugin\TestPlugin.uplugin). Plugin names must be unique.

It's finding the temporary duplicate of the .uplugin while it's being used in HostProject and stopping the build. In previous engine versions I don't believe any errors were thrown when a .uplugin existing in \\Engine\Plugins\... was found with the same name as the .uplugin you're calling BuildPlugin on somewhere else.

Steps to Reproduce

1. Ensure that the .uplugin of a plugin outside the \\Engine\Plugins\... folder has the same filename as a .uplugin inside the \\Engine\Plugins\... folder.

2. Call BuildPlugin on the .uplugin (the one outside the Engine folder), potentially with a .bat that looks something like:
CALL "C:\Program Files\Epic Games\UE_4.20\Engine\Build\BatchFiles\RunUAT.bat" BuildPlugin -Plugin=C:\PluginDir\PluginToBuild -Package=C:\PluginDir\BuiltPlugin -Rocket -targetplatforms=Win64

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Fix Commit4419034
Main Commit4450599
Release Commit4419034
CreatedOct 2, 2018
ResolvedOct 3, 2018
UpdatedOct 26, 2018
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