I work with 5 Excel sheets but opened at the same times

The information on each sheet is as follows
an example

Sheet1
A1 ULD number as PMC12121KZ
B1 Weight as 1200Kg
C1 Height as 160cm
enz

and so several rows under each other

All the information on sheet 1 MUST be unique so if the same information
will be inputted on sheet number 2 as
A1 PMC12121KZ a warning must be given and a choice I must have to import
on sheet number 2 with all relevant information as column B,C,D,E enz or
leave on original sheet

How do I start this?

Stefan