I see double lightboxes

In case clicking on a thumbnail opens two lightboxes, then one of them is coming from JIG and the other is what you already use. It's most likely added by your theme, and on some occasions by a 3rd party lightbox plugin. You need to decide which one to keep, after you've identified them. Learn more about JIG's lightboxes on the Which lightbox to use? page.

• Find a way to get rid of the other lightbox. If it's coming from a plugin, then just disable / uninstall the other plugin.
• The theme should have an option whether or not you intend to use its lightbox. If you can't find it, ask the support of the theme regarding disabling the lightbox.
• In most cases, it's possible to add JIG elements as exceptions regarding what your existing lightbox is used on. However, it might require a little custom coding. It will allow you to use the existing lightbox as a fallback for incidental single pictures, while letting JIG use its own, different lightbox. This is an example of how to prevent the other lightbox from getting opened from clicking JIG thumbnails:
  
  jQuery('a[rel*="lightbox:not(.justified-image-grid a)"]').fancybox();

  1	jQuery('a[rel*="lightbox:not(.justified-image-grid a)"]').fancybox();

  Or slightly differently:
  
  jQuery('a[rel*="lightbox"]').not('.justified-image-grid a').fancybox();

  1	jQuery('a[rel*="lightbox"]').not('.justified-image-grid a').fancybox();

  Observe the use of quotes and the difference between :not() and .not() when using jQuery. In this example it's called fancybox and the selector is based on a rel value, but other scenarios are similar. Change this where the theme applies its lightbox, often in custom.js or main.js found at a location like /wp-content/themes/Avada/js/.
• In certain plugins, such as Responsive Lightbox by dFactory there is a setting for the lightbox selector rel, so just use the core of the solution from above:
  
  lightbox:not(.justified-image-grid a)

  1	lightbox:not(.justified-image-grid a)

• Please be advised that JIG will not completely replace the functionality of your theme's lightbox (handling single pictures in content). Can I use a JIG lightbox for other single images on the page?

• For the JIG settings > Lightboxes tab > Lightbox type setting, choose Custom: I already use a lightbox plugin so I'll set up the link class and/or rel accordingly option. This will disable the built-in lightbox for desktop. If you use presets in the shortcodes, please look into Custom presets and authority as presets bring their own lightbox choices by default.
• Do the same thing for the Mobile lightbox setting as well.
• If you feel that JIG is not using your custom lightbox well enough, look into JIG settings > Lightboxes tab > Custom lightbox JS call setting.