finding repeated data sequences in a column

[yadin]

Good day everyone!
I have a a table, from where I can extract a column.
I wanna go down trough that column made of numbers
examine undetermined chunks of data and see or detect if that sequence
of chunk
of data has been repeated before
and if it has been repeated detect it by giving it a name in an
adjacent column.
Imagine someting like this but made of 1800 numbers... how can I build
up column 3(category)

Item	price	          Category
400	1000028706	A
400	1000028707	A
400	1000028708	A
101	100	                C
101	12	                C
500	1000028706	A
500	1000028707	A
500	1000028708	A
500	1000028709	B
120	1000028706	A
120	1000028707	A
120	1000028708	A
120	100	                C
120	12	                C