There are supposed to be 44 Common Item Investigator Cards but I just have 43. Which one am I missing. Is it the Lucky Cigarette Case? I have just one of those and two of everything else. Could someone please confirm this please.

Yup, there should be two Lucky Cigarette Cases.

Here's a reference for all the cards:

ArkhamHorrorWiki-commonItmes

I'm almost positive it's the lucky cigarette case. 99% that is.

Check the component lists on BGG

One thing worth checking that I found out when I did a complete check of all my cards (at the same time as removing all expansions out of the base game).

I thought I was missing one spell card from dark paroah (I triple checked it, and then searched all the other seperated decks again).

Turned out it was stuck to the back of another card (making it slightly thicker). With the volume of cards it was easy to miss this.

So for what its worth it might be worth rechecking the common item decks in case one card is just a bit thicker then it should be before sending in for a replacement.