voiceless voiced voiceless voiced
Stops Double Stops
p b pp bb
t d tt dd
c, k g ck gg
Fricatives Double Fricatives
s z ss zz
f v ff Ø
θ ð th th
ʃ (t: -tio-) ʒ (j, s: -sio-) sh Ø
Liquids & Nasals Double Liquids & Nasals
Ø l Ø ll
Ø r Ø rr
Ø m Ø mm
Ø n Ø nn
Ø ŋ Ø ng

Usually, the purpose of doubling these consonants is to clarify that the vowel before is short.

Stops have two sounds, one when before a vowel, one after the vowel—a "stop" and a "start" version. Doubling a stop in a spelling usually indicates pronouncing both the "stop" and "start" sounds. Examples are: bubble, daddy, wiggle, happy, little, and chicken.

For non-native speakers...

Practice stops by sandwiching them between vowels and repeating them as an exercise. For example with "b": bababababababa. Do this with all Stops.

Practice "l", "th", and all "l" combinations by sticking your tongue out, relaxed, the entire time. Practice "bl", "fl", "pl", etc. by sticking your tongue out the entire time, even spitting like a baby would.

If "l" is a problem, which it can be for some far eastern speakers, practice "l" by putting the tip of your tongue behind your lower front teeth and pronouncing it with the middle of your tongue against your top front teeth.