Does CASL require single or double opt-in?

Generally no, but double opt in is a good idea. See below

http://www.slideshare.net/marketo/getting-ready-for-the-canadian-antispam-legislation?ref=https://www.marketo.com/

http://www.slideshare.net/marketo/eprivacy-law-marketers-need-to-know?ref=https://uk.marketo.com/

blog.marketo.com/2014/03/stay-in-compliance-what-marketers-need-to-know-about-eprivacy-law.html

marketo.com/cheat-sheets/a-marketers-guide-to-canadas-anti-spam-legislation-casl/

This is what I've figured out (not legal advice):

• Exceptions to Explicit Opt In
  • someone asked for help
  • between organizations with a relationship
  • notice of legal rights (contracts)
  • notice of product
• Referrals
  • Sender can send 1 message and make note of who referred to Recipient A
  • Recipient A and Referrer should have relationship
  • Referer and Sender must have relationship
• No Consent Required for
  • Providing a quote
  • confirming a transaction
  • providing warranty, recall, safety, or product recall info
  • Ongoing use of a product or sub
  • delivers goods, updates, or upgrades
• Implied Consent
  • Existing relationship - within 2 years of purchase
  • within 6 months of inquiry
  • Conspicuous Publication (vague)
  • email or phone provided by recipient with above restrictions
• Explicit Consent (this is the part you asked)
  • opt in, no precheck - but ok to just have email address if it’s clear they are opting in.
  • must say you can unsub
  • must identify self and address
  • must be clear they are opting in.
• Proving Consent
  • Log Date Time
  • IP Address
  • URL and Form
  • retain image of forms