To be short, they are the same, either one will work. It means understand.
Most Chinese vocab is built by a combination of 2 individual words. For example, ming4, can be used to form lots of vocab. e.g. ming4 loeng2 means bright, ming4 hin2 means obvious, ming4 baak6 means understand.
When you use the word alone, ming4, it already means understand by default, already imply ming4 baak6, so you can say it is a short form. Same idea can be applied to zi1 dou3. It means to know. For I know, you can either say ngo5 zi1, or ngo5 zi1 dou3. It implies.
This idea not only works on the first word. For example, sik1 means to know someone, or some skills. You can either say jing6 sik1 or simply sik1. Because sik1 already implies jing6 sik1. For example, Ngo5 sik1 Stephen.