Use of "would" for subjunctive phrases
Solution 1:
Practical English Usage, which is a pedagogic grammar for English learners, states:
Conditional would is sometimes used in both clauses of an if-sentence. This is very informal, and is not usually written. It is common in spoken American English.
- It would be good if we'd get some rain.
- How would we feel if this would happen to our family.
Your original sentence (If that would happen, you won’t be alone) combines would in the if-clause and future with will in the main clause, which is ungrammatical.