You can remember the meaning of 不同 with this easy mnemonic:
When my twin brother and I were little, there was no (不) way to figure out which one of us was which since we looked exactly the same (同). One had to pay close attention to what we wore to figure out that our necklaces were different.
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.