Here's another post from Matt:

I just want to clear a few things up on the duplications rule:
  • Units of the same type that can be selected from different slots do not count as duplications. This is to prevent it becoming too complicated, such as command squads being counted as the same as veteran squads etc.  
  • Units that can have a mixture of different types of models are always duplications if two are taken. For example a leman russ squadron is a duplication of another leman russ squadron even if the first squadron contains battle tanks and the second contains demolishers as each unit could contain a mixture. 
  • Astra militarum platoons count as a single unit choice, and so two platoons may be taken 
  • However the max squad size rule still comes into play during the game and so combined squads of guardsman can never be over twice their starting size (so only two squads can be combined to make a max size of 20 guardsmen) 
  • Models selected from a single choice within a codex (e.g. 0-3 per.... etc) only count as a single choice per 0-3 selected. 
Below is an example of an Astra Militarum list:

Combined Arms Detachment - 1850pts
Priest
Priest
Primaris psyker ML2

Tank Commander (Pask) in Punisher with another Punisher (both with HBs and lascannons)

Tempestus Scions, 5 man with 2 melta

Infantry Platoon
2 infantry squads both with lascannons
Platoon command with 4 flamers

Infantry Platoon
2 infantry squads one with lascannon, one with autocannon
Platoon command

Veterans (demolitions) in Chimera with 3 melta

Vendetta

Leman Russ squadron – 2 Executioners with plasma sponsons and lascannons

Wyvern Battery – 2 Wyverns

The only duplications are the infantry platoons. The Leman Russ are not duplicated as they are in different slots. The priests do not count as a duplication as they are selected as a single choice (0-3). Therefore 3 could be taken as a single choice if desired. I would combine the infantry squads into two 20 man squads with priests and a psyker in one to hopefully cast Prescience on my Executioners. Platoon Command with flamers will go into the Vendetta and the other will support the infantry squads.

Hope this helps with list writing!

Matt