Tracing a Session of Particular Id

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.NET

C#3.5

I am working on Desktop application and Using RFID Reader which is like any bar-code reader which read TagId from Tags when a Tag is Placed in front or in Range of reader it start reading that Tag and store value in database (Supppose, 1AC7B this is TagId which reader is reading, it read this tag continuously when Tag is in range of RFID reader and stop reading it when it move away , Reader read TagId per second) Now each
time it read Tag and add this TagID to ListBox with current Date and Time

Listbox
1AC7B
1AC7B
1AC7B
1AC7B
now i want to trace the duration of Particular Tag Id means when Tag comes in range of RFID reader and move away from the reader so please help me how can i achieve this
1 idea is that store TagId with datetime in database but this fails because when Tag again come in range of reader it again start reading TagId and store this in database this cause ambiguity so please help me how can i achieve this via programming so to trace the time duration when Tag is in range of Reader.