Are these sentences OK?
We are taking the necessary measures not to happen something like this again.
We are taking the necessary measures not to happen such a tragedy again.
We should take the necessary measures to prevent it from happening again.
We should take the necessary measures to prevent any further tragedies like these.
We have taken every measure to see it doesn't happen again.
We have taken every measure to prevent this sort of thing from happening again.

Question

Answer

Same as above. The term ‘to ensure’ (to make sure, to be certain) is often used in sentences like the first two:

We are taking the necessary measures to ensure that...

...nothing like this happens again
...a tragedy like this never happens again

Answer

1. This should be: We are taking the necessary measures not to let something like this happen again.
2. This should be: We are taking the necessary measures not to let such a tragedy happen again.
3. This is correct.
4. This is correct.
5. This should be: We have taken every measure to make sure that it doesn't happen again.
6. This is correct.