Using wp single login you can easily add the functionality to allow only single login per user.
More Plugins
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Widgets on Pages
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UsersWP – Front-end login form, User Registration, User Profile & Members Directory plugin for WP
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Photo Reviews for WooCommerce
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Multiple Shipping And Billing Address For WooCommerce
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Menu Duplicator
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Plugin Organizer
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Agreement
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GlotPress
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Brozzme DB Prefix & Tools Addons
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Kama Thumbnail
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Checkout Field Editor (Checkout Page Manager) for WooCommerce
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Fix Another Update In Progress
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Hotel Booking for WooCommerce
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wpautop control
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Sticky CTA – Sticky Floating Notification bar, Buttons, Call To Action A/B Testing
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Simple Yearly Archive
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Disable Payment Methods based on cart conditions for WooCommerce
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WP Force SSL & HTTPS SSL Redirect
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User Profile Builder – Beautiful User Registration Forms, User Profiles & User Role Editor
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Quiz and Survey Master (QSM) – Easy Quiz and Survey Maker