Postdoc: internal v. external candidates. Can I expect a fair process?
Nothing in life is certain, of course. Different places have different rules and different levels of actual adherence to the rules. Your concern that they may prefer a known quantity may well be balanced by their desire to bring in a fresh face. Different members of the search committee might have different preferences if they have any at all. 

But it sounds from what you write that you are likely in a fair process even if that isn't a universal phenomenon. 

But, in the end, they will choose the person who they feel is best for the position, based on whatever formal and informal criteria they have. You have no actual influence over that. 

I'd suggest that you take a deep breath and also have a backup.