Using wp single login you can easily add the functionality to allow only single login per user.
More Plugins
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Minimal Share Buttons
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Shadowbox JS
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FV Clone Screen Options
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Flexible Posts Widget
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WP Inline Edit
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Disable WYSIWYG
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Smartarget – Chat Buttons & Engagement Apps
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Multiple Domain Mapping on Single Site
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Zeno Font Resizer
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Software License Manager
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HTML5 Maps
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VRTs – Visual Regression Tests
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GutenKit – Page Builder Blocks, Patterns, and Templates for Gutenberg Block Editor
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WowStore – Store Builder & Product Blocks for WooCommerce
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Sale Price for EDD
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Gutenberg Block Editor Toolkit – EditorsKit
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RSS for Yandex Turbo
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Bulk Edit Easy Digital Downloads – Fast Bulk Creator
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ACF to Custom Database Tables
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WP-LESS