Using wp single login you can easily add the functionality to allow only single login per user.
More Plugins
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Helpie FAQ — Accordion, Docs & Knowledge Base
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Faview – Virtual Reviews for WooCommerce
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Kama WP Smiles
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Comment Approved
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Kirki – Freeform Page Builder, Website Builder & Customizer
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Quotes llama
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FluentCart A New Era of eCommerce – Faster, Lighter, and Simpler
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Newsletter – Send awesome emails from WordPress
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Marquee Running Text
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Fluid Player
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WP Keap/Infusionsoft WooCommerce Plugin
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PDF Invoices Italian Add-on for WooCommerce
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OTF Regenerate Thumbnails
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ThirstyAffiliates – Affiliate Links, Link Branding, Link Tracking & Marketing Plugin
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Vanilla PDF Embed
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Simple XML Sitemap Generator
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Social Viral Downloader
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WP-Paginate
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Affiliate Power – Sales Tracking for Affiliate Marketers
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WP-KaTeX