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I'll be glad to answer your questions, but suggest you click the FM Pro Forum tab at the top of this screen if you have any additional questions to ask about the product as questions posted there will be seen by many more people and thus will gather more responses.
1) The best way is to not permit them in the first place with a good interface design, but you can set a unique values validation rule that can pop up a message you specify when a duplicate value is entered into a field where you've specified this validation rule. You can also devise scripts that perform more sophisticated validation checks when the "onObjectValidation" script trigger is tripped.
2) Can't really answer that question. I can think of many ways to do that and the best answer depends on what you need to update. It could be as simple as a script that finds that records so that the user updates it directly, but there are many other options possible as well.
3) In FileMaker, you can add a block of new records to an existing table by using the Import Records Action from the File Menu or via a script that uses the same method.
If you have not already done so, I recommend downloading the 30 day free trial to get direct, no cost experience with this product.
If you then decide to go ahead and purchase it, I strongly recommend that you acquire a copy of FileMaker Pro Advanced for developement work. It offers a number of development support tools (including script debugger and data viewer similar to the ones that come with Access), that can save you a lot of time and effort developing a database as well as gaining the ability to take more complete control of the user interface with custom menus and custom functions.
One final note: FileMaker is not Access. It does not allow the user or developer to use SQL to produce recordsets for forms and reports the way you are used to in Access. It has it's own unique way of accomplishing many of the tasks for which you would use SQL in that system so you have to adjust your thinking and reach for a very different set of tools in order to accomplish the same thing as you would in Access.