Using wp single login you can easily add the functionality to allow only single login per user.
More Plugins
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WPB Custom Tab Manager for WooCommerce – Add, Edit & Reorder Tabs
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Table rate shipping for WooCommerce
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Packlink PRO for WooCommerce
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WP Event Calendar
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Meks Easy Social Share
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Geo Data Store
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Add Polylang support for Customizer
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Malcure Malware Shield — Removal, Repair, Monitor
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Disable Recaptcha – CF7
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Single Author G+ META
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GTmetrix for WordPress
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DC Simple Share Buttons
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Social Media Auto Poster – Schedule & Publish to Buffer
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Advanced Custom Fields (ACF®)
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Product FAQ for WooCommerce
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Photoswipe Masonry Gallery
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Redis Object Cache
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Requird
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Epoch – A native Disqus alternative with a focus on speed and privacy
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Author Spotlight (Widget)