The difference between the chance of a single 12-word seed being made twice and the chance of a single 24-word seed being made twice is approximately ~2.93 * 10^-39. Although technically about a bitcoin private key, and not an electrum seed, the image that bitmover posted nicely illustrates the risk of a collusion.

---

It is important to make sure that your computer is capable of generating random numbers. In 2013, for example a flaw in Android devices prevented them from generating cryptographically random numbers, which resulted in the risk of malicious actors being able to steal any coin stored on Android devices. The above did not ever affect electrum.

Today, most computers and mobile devices can generate cryptographically random numbers. However, if your device is infected with malware, the malware may cause your computer to generate numbers in a predictable manner, which could lead to a hacker stealing your money, even if you generated the seed on an offline computer (that was infected with malware).