Most cars are different, but there are some examples where the cars is the same, to even the finest detail. I'll give you a bunch of examples:
Audi R8 LMS '09
Audi R8 LMS (Team PlayStation) '09
Is exactly the same.
Honda TAKATA DOME NSX '06
Honda RAYBRIG NSX '06
Honda ARTA NSX '06
Are all exactly the same.
Honda HSV-010 GT Base Model '12
Honda RAYBRIG HSV-010 '12
Honda KEIHIN HSV-010 '12
Are also all the same, its only the livery that differ, with the base model having no livery at all.
Honda weider HSV-010 '11
Is NOT the same as the other 3 HSV-010 cars.
Nissan GT-R NISMO GT3 N24 Schulze Motorsport '13
Nissan GT-R NISMO GT3 '13
Nissan GT-R NISMO GT3 Base Model '13
Is exactly the same, while the:
Nissan NISMO GT3 Nissan GT Academy Team RJN '13
Is NOT the same!
Honda S2000 (US) '01
Honda S2000 (EU) '01
Are both identical.
Honda S2000 Type V (EU) '01
Honda S2000 Type V (US) '01
Are both identical aswell.
Honda S2000 Type V (US) '00
Honda S2000 Type V (US) '01
Are NOT identical!
Mazda MX-5 Miata VR-Limited (NA, J) '95
Mazda MX-5 VR-Limited (NA, J) '95
Are the same, and
Mazda MX-5 Miata J-Limited II (NA, J) '93
Mazda MX-5 J-Limited II (NA, J) '93
are the same aswell, and ALL 4 are identical in specifications, down to every little detail. However, they still differ a bit when driving them. (the first 2 differ slightly from second 2.)
Toyota 2010 Brian Vickers #83 Red Bull TOYOTA CAMRY
Toyota 2010 Joey Logano #20 The Home Depot TOYOTA CAMRY
Toyota 2010 Kyle Busch #18 M&M'S® TOYOTA CAMRY
Toyota 2010 Denny Hamlin #11 FedEx TOYOTA CAMRY
Ford 2010 Carl Edwards #99 Aflac FORD FUSION
Ford 2010 AJ Allmendinger #43 Best Buy FORD FUSION
Chevrolet 2010 Dale Earnhardt Jr. #88 AMP Energy/ National Guard CHEVROLET IMPALA
Chevrolet 2010 Jimmie Johnson #48 Lowe's CHEVROLET IMPALA
Chevrolet 2010 Juan Montoya #42 Target CHEVROLET IMPALA
Chevrolet 2010 Jeff Gordon #24 DuPont CHEVROLET IMPALA
Chevrolet 2010 Tony Stewart #14 Office Depot/Old Spice CHEVROLET IMPALA
Toyota 2011 Brian Vickers #83 Red Bull TOYOTA CAMRY
Toyota 2011 Kyle Busch #18 M&M'S® TOYOTA CAMRY
Toyota 2011 Joey Logano #20 The Home Depot TOYOTA CAMRY
Toyota 2011 Denny Hamlin #11 FedEx TOYOTA CAMRY
Ford 2011 Carl Edwards #99 Aflac FORD FUSION
Ford 2011 AJ Allmendinger #43 Best Buy FORD FUSION
Chevrolet 2011 Jimmie Johnson #48 Lowe's CHEVROLET IMPALA
Chevrolet 2011 Juan Montoya #42 Target CHEVROLET IMPALA
Chevrolet 2011 Jeff Gordon #24 Drive to End Hunger CHEVROLET
Chevrolet 2011 Tony Stewart #14 Office Depot CHEVROLET IMPALA
Chevrolet 2011 Dale Earnhardt Jr. #88 AMP Energy/ NG CHEVROLET IMPALA
Are ALL exactly the same!
Toyota 2013 Matt Kenseth #20 Dollar General/Home Depot TOYOTA CAMRY
Toyota 2013 Kyle Busch #18 M&M'S® TOYOTA CAMRY
Toyota 2013 Denny Hamlin #11 FedEx TOYOTA CAMRY
Ford 2013 Carl Edwards #99 Fastenal FORD FUSION
Ford 2013 Aric Almirola #43 Farmland FORD FUSION
Chevrolet 2013 Jeff Gordon #24 Drive to End Hunger CHEVROLET SS
Chevrolet 2013 Juan Montoya #42 Target CHEVROLET SS
Chevrolet 2013 Jimmie Johnson #48 Lowe's CHEVROLET SS
Chevrolet 2013 Dale Earnhardt Jr. #88 National Guard CHEVROLET SS
Chevrolet2013 Tony Stewart #14 Bass Pro Shops CHEVROLET SS
Is also believed to be exactly the same as the 2010/2011 nascar's, but not confirmed yet.