I want to know, we are updating to Magento 2, do we need to pay for the extensions we bought on magento 1 if they are available on magento 2?
TL;DR: This does not depend on magento. It depends on each extension provider.
Magento 1 and 2 are 2 different platforms.
And so are the extensions with the same functionality.
I guess it depends on how the extension providers want to distribute their modules for Magento 2.
Most likely you will have to pay for the new magento 2 modules unless the extension provider has a policy to give you free upgrades to M2 modules.
Also, keep in mind that some of the extension providers might not have a M2 equivalent for an M1 extension.