Using wp single login you can easily add the functionality to allow only single login per user.
More Plugins
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Multipage
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Add Logo to Admin
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Contact Coldform
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WP Change Custom Posts Slugs
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FancyTube – Video Gallery, Video Slider, and Playlist Slider for YouTube
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Revisionize
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Postname Permalink Auto Redirect
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Multisite Site Index
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Booster for WooCommerce – PDF Invoices, Abandoned Cart, Variation Swatches & 100+ Tools
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Collapsible Widget Area
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Admin Bar Edit Content Links
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WC Duplicate Order
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WCFM Marketplace – Multivendor Marketplace for WooCommerce
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Product Options and Price Calculation Formulas for WooCommerce – Uni CPO
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Fix Category Count
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Advanced Image Styles
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Attribute Stock for WooCommerce – Shared Stock & Variable Quantities (Lite Version)
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WP Dummy Content Generator
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Kadence Security – Password, Two Factor Authentication, and Brute Force Protection
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Metro Style Social Widget