Howdy @Sophie Wang,
These are all great questions! Let me first preface saying that all the information provided in my comments below are only for Florida tax certificates/liens as every state is different. I assume you will be bidding on Florida certificates, but saw you are in Colorado so just making sure. Please see my comments below your questions and let me know what else I can help you with.
1. Does an investor get priority right to purchase subsequent years of tax liens after he/she made the initial purchase? Will the interest rate for sebsequent years 18% or whatever rate the investor bid for the first year's lien?
Each year is treated as a new certificate so to answer your first question, no; you can buy 2017's certificate, but next year anyone can bid for 2018. The subsequent taxes (any taxed unpaid after your purchase) are free game and will not affect your certificate - unless of course they file for a tax deed after two years and pay your certificate off - yay!
2. Does florida allow multiple liens on one property? For example, there is a property, investor A purchased tax liens on it for year #1, but did not want to purchase the lien for year #2, in this case does the county bundle the lien plus interest for year #1 with the amount for year #2 and offer it for bidding again in year #2 and be purchased by investor B, and investor A got redeemed, or does the county allows A and B hold liens on the same property for different years?
The later situation you stated is correct. The county allows both investor A and B to hold separate liens (certificates) on the same property for different years. Investor A will keep collecting the original said interest rate until the certificate is paid. Investor B will collect their interest rate (whatever they bid) until their certificate is paid.
3. If a property has liens purchased by investor A and B for different years, do A and B have the same right to force foreclose when the 2 year redemption period is met? Either of them can pay off the other and foreclose? Can the lien holder bid at the foreclosure auction too if he or she wants the property?
Yes, investors A and B both have a right to apply for the tax deed (which will initiate the auction to foreclose on the property as you stated). The difference is if Investor A purchased the tax year ahead of Investor B, then Investor A can apply for the tax deed one year sooner. Investor B has to wait their full two years from the "purchase date" of the certificate as well. Either of them can pay off the other and foreclose. Now, Investor A may not want to apply for the tax deed right at two years and they have the right to wait up to 7 years from purchase date to apply for the tax deed. During that time, any of the other investors after that can apply and essential will pay off Investor A's certificate. During that time, Investor A will keep the same interest rate each year. Interesting side note, if you research any properties that have expired tax certificates, where the lien holder did not apply for the tax deed within 7 years, this is a HUGE red flag that the property is not worth it.
For your last question, yes, anyone can bid on the property during the auction. The Investor who applied for the tax deed basically starts the auction at the minimum price since they have paid for all the past due taxes from other years, plus all the fees. If no one bids against them, they automatically get the property. This RARELY happens unless it's a useless piece of land. A good example would be to go back and look at the Pinellas County tax deed auctions for any land in St Petersburg on the South Side. Another example is Hernando County, where Brooksville and Weeki Wachi have a bunch of developers that go bankrupt and land is not bid on since it's not worth the starting bid. If you do want to bid on the property once it goes up for auction, this is a separate process and website from the annual tax certificate sale. You will need to sign up for an account and make a minimum deposit before bidding.
Hope this helps. Let me know if you have any further questions. Cheers.