Few weeks ago I wrote most popular reasons of this problem: direct connection between head tags, use low-quality or old themes/plugins, using different IDs in wp_enqueue_script function.
This simple plugin demonsrate the SLON technology (more about SLON technology). You can choose, from where load Bootstrap for your site – from your site (plugin directory), from BootStrap CDN, or dont load BootStrap (in case, if BootStrap load via your theme).
Example 1, Bootstrap will load from Max CDN:
Example 2, Bootstrap will load from your site (from plugin directory):
1. Upload the plugin files to the /wp-content/plugins/wa-slon-example directory;
2. Activate the plugin through the ‘Plugins’ screen in WordPress;
3. Use the “WA plugins > SLON Example” menu and choose, from where load Bootstrap;