I checked the Nellie and Joe's Famous Lime Juice
website and there is no mention of their lime juice being double-strength. In reading their FAQs
page, there is also no indication that it is double strength. In fact - it would indicate that it is not double strength. They show that for their lime juice: 1-2 Tablespoons = the juice from 1 key lime, 2-3 Tablespoons = the juice from 1 regular lime. If you compare this to ReaLime lime juice, which is just regular full strength, 2-3 Tablespoons = the juice from 1 medium lime, and on a volume basis is the same as fresh squeezed lime juice (ie. 1 C = 1 C).
Nellie and Joe do have a "Contact Us" button you can click and send them an email with your questions, or you can call them at 1-800-LIMEPIE for a quicker answer.
Hope this information helps.