Data lost after INSERT command

I have a website that is used to enter dates from a calendar. For example, user A might go to the website and select 20 different days in January. When user A clicks on the Save button the dates are written to the database with an INSERT command. This works fine. However, when user B then goes through the same procedure, say selecting 5 dates that are in the set selected by user A, those dates are properly saved in the database but the rows containing user A's same 5 dates disappear. I suspect this is something simple but I'm new and ignorant. Please help.Are you sure that when User B selects dates that are in the set selected by User A you are not using an Update statement instead of an Insert statement?|||It would be very helpful if you would post the queries involved, as well as the table DDL.|||Thank you for taking the time to respond. I have a bit of code where I do a DELETE prior to doing the INSERT. Your question led me to examine my DELETE statement again, only to discover that I had made it too broad so that it was deleting all records having a particular date, not just the one of the user currently logged in. I will adopt the suggestion to include code in future postings. Thanks again.