I'm not watching it but, if the scheduled programme is being shown (rather than, say, a last minute replacement because of some sort of problem), it should be Season 12, Episode 6, which is summarised as follows:
"Davina McCall and Nicky Campbell join forces with investigators from the MOD's Joint Casualty and Compassionate Centre to try to trace the families of nine First World War soldiers whose bodies were found together in a trench by roadworkers in Beselare, Belgium".
If that's what you're seeing, it theoretically should be a new programme, as Series 12 only commenced airing on 13 June.
However Wikipedia appears to show that it (or something very similar) was actually previously aired, as a one-off special, on 21 Oct 2019:
https://en.wikipedia.org/wiki/Long_Lost_Family_(British_TV_series)#Regular_series
. . . whereas the Commonwealth War Graves Commission website refers to "Following the success of the first special shown in late 2017 . . . ".
https://www.cwgc.org/our-work/news/long-lost-family-the-unknown-soldiers/
So it seems that either the programme as a whole has been shown before or the current episode has been spun off from an earlier special.