Using wp single login you can easily add the functionality to allow only single login per user.
More Plugins
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Brozzme DB Prefix & Tools Addons
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Kama Thumbnail
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胖鼠采集(Fat Rat Collect)
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Checkout Field Editor (Checkout Page Manager) for WooCommerce
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User Role Sync for MailPoet
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Fix Another Update In Progress
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Simple Yearly Archive
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IfElse Pages – Coming Soon and Maintenance Mode
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Gallery without the Fluff
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WP Force SSL & HTTPS SSL Redirect
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Contact Us for WP
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Embed Images in Comments
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User Profile Builder – Beautiful User Registration Forms, User Profiles & User Role Editor
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bbPress Enable TinyMCE Visual Tab
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Quiz and Survey Master (QSM) – Easy Quiz and Survey Maker
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Media Library Downloader
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Smart Coupons For WooCommerce Coupons
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Cool Timeline (Horizontal & Vertical Timeline)
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Text Control
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Events Search For The Events Calendar