craft 3 finding plugins from two places instead of one

Description

After converting Trevor Davis's videoembed, I was experimenting with namespaces etc. vs. Composer to try to get folder name craft3-pluginname acceptable for install. I had this working with the bare plugin namespace (no hyphens) plus a composer.json path pointing to craft3-pluginname.

What happened then on the craft 3 plugins page is that I then showed two copies: one from the /plugins/craft3-pluginname folder, and one from the /vendor/signature/pluginname folder, both showing the plugin name from the acceptable non-hyphenated namespace (plain videoembed). Thus it seems Craft is improperly searching both paths to identify plugins.

Add'l notes: The two folders are separate on my Vagrant dev server, because I told rsync to make copies rather than preserve the Composer symlink, due to different filesystems path heads. The site itself is freshly converted at beta 9, from Craft 2.

Steps to reproduce

1. As above
2. I'm actually running Craft:dev/develop, which points at just released beta 10 at moment

Additional info

PHP version 7.1.3-3+deb.sury.orgxenial+1
Database driver & version MySQL 5.7.17-0ubuntu0.16.04.1
Image driver & version GD 7.1.3-3+deb.sury.orgxenial+1
Craft edition & version Craft Personal 3.0.0-beta.10
Yii version 2.0.11.2
Twig version 2.1.0
Guzzle version 6.2.1
Imagine version 0.7-dev
Plugins

Bactrian 1.0.0
Trimmer 1.0.0
Standards Agent 2.0.4
Video Embed 3.0.0

Start by reproducing the Craft 3 beta installation with the plugin directory and Composer vendor directory both present, using the paths described in the issue. Trace how the plugins page discovers plugins and verify that the same plugin is not listed twice when those two locations contain copies of it.