What is the best approach to update the custom symbol's historical data in MT 4 and 5?

[Sebastianucas]

Hello to all of you. I have a brief inquiry:

I now want to regularly update the historical data in MT, perhaps once a month or once a quarter.
I would like to know how to update the tick/bar data of a custom symbol in MT, given that my two new CSV files also include the historical data that was previously imported into MT. (For example, I imported all historical data up to September 30th before; the CSV files include all historical data up to December 31st for the upcoming upgrade.)
If there is just one way to change the custom symbol, is it to import the two ticks & bars CSV files, make a new custom symbol for every "update", and then remove the previous custom symbol?
Or is it possible for me to "overwrite" the entire history data of the symbol with my new import?
I want to prevent MT from having duplicate ticks or bars from the past.
Is it appropriate for me to import the CSV files without any data linked with the custom symbol? Stated differently, can MT4 and MT5 recognize duplicate data that it has already encountered during import?
(Or do I need to go through my new CSV files and manually remove every tick and bar that MT already has? By the way, is it possible to view the most recent timestamps of the bars and ticks that MT already has for a customized symbol?

If someone who has previously updated custom symbols could confirm the proper approach (for both MT 4 and 5), that would be really appreciated.