In Litecoin every single block is mined every 2.5 minutes, double spending? In Bitcoin it takes roughly 10 minutes for every block to be mined.
In Litecoin it takes 2,5 minutes.

2.5 minutes in Litecoin mean that transactions can be confirmed faster.
In Bitcoin these 10 minutes are not  coincidentally, it has been choosen also to prevent double spending.

Why does Litecoin allows 2,5 minutes and not, like Bitcoin, 10 minutes? 
Is the probability of having double spendings in Litecoin not much higher when using 2,5 minutes ?
The probability is not dependent on the block time alone, it is dependent on the block time, and how many blocks you wait before considering a transaction complete.

For Bitcoin, generally 6 blocks is considered to be the point after which a chain reorganization that changes the block the tx is in is considered to be impractically due to the cost involved with gaining enough hashpower to roll back 6 blocks.

Due to this, most exchanges and merchants will require 6 confirmations before crediting your account (although, Bitcoin is by far the largest network in terms of mining power, so for certain use cases, even 2-3 confirmations can be considered enough).

Litecoin has a lower block time, and to guarantee the same security, receivers should wait for a greater number of confirmations before considering the deposit complete.

Ethereum, with a block time of 15 seconds, is even faster. Thus, many exchanges will wait for even more confirmations (I've seen up to 50 for ETH and token deposits).

The block time alone does not guarantee safety against double spends. Even a 10 minute block-time chain can be double spent if you only wait for a single confirmation.  To prevent double spends, you simply need to find the tradeoff point after which it is impractical for someone to roll back the chain.

In most cases, the actual confirmation count used is often less than this impracticality point, since for smaller chains it may require too many confirmations, leading to a usability issue. Thus, exchanges may settle for a lower number of confirmations than required for absolute safety, but still good enough to guarantee it in the vast, vast majority of cases.