Question about debian install & multi OS.
 

I did a new install of debian 10.4 but did not use LVM just the full hdd install but will this still recognize a second OS if i try to install one ?

yes

Thank`s...One other question, at what stage will i be notified that a OS is already installed just so i know / I don`t want to delete the current one by mistake...Cheers.

It depends on the distribution and its installer.

Ubuntu and its derivatives will detect another operating system at the partitioning stage and have an install along side option. They typically will be able to shrink an existing partition to create free space.

Others will not. They expect free space to already exist and you must use the manual partitioning option. Be aware the guided or automatic option will typically use the first drive and overwrite everything. There are usually enough warnings that you can exit safely if you are careful.

The main stream distributions have installation guides. It advisable to read it before installing.

Of those installers that detect another OS will typically add it to their grub configuration others may not and you have to fix after installation.