First some bottle comparisons for Can't Find my Czechbook.
I suggest zooming in on this photo! L-R is China Glaze Sea Spray, Nubar Faded Jeans, OPI Czechbook, butter London Artful Dodger, Dior St Tropez, China Glaze For Audrey, Chanel Nouvelle Vague and OPI Fly.
I thought Czechbook would be far more green/turquoise, but upon closer inspection it is much bluer.
As you can see it's dupe free in my collection, closest to Nubar Faded Jeans and butter London Artful Dodger. It is much brighter than the others I own.
Next is OPI...Eurso Euro
L-R is China Glaze Ride the Waves, Dior Blue Denim, OPI...Eurso Euro, Dior Blue Rebel and OPI Ski Teal We Drop
I got rid of Ski Teal We Drop as it was far too green. Dior Blue Denim was the closest match, but it has a full creme consistency whereas Eurso is half jelly, half creme. Ride the Waves is nothing like it and is too sheer. Blue Rebel is also too green. I don't have any other royal blues, but possible Revlon Royal or OPI Dating a Royal or Ogre the Top Blue might be in the same family? Let me know if you think you have a dupe in the comments.