This is a very common mistake but it's also one that's easily corrected and learned. If you incorrectly say "did I told you" you have two words that are in the Past Simple Tense: "did" and "told". You don't want to have a double past tense. When you ask questions and you reverse the word order, you have to make sure that you're not doubling up. When you use "did", make sure you use the infinitive form of the verb, actually a bare infinitive (without "to"). I told you. Did I tell you? We went there. Did we go there? They saw that. Did they see that? While we're on this topic, a similar mistake is often made with a third person singular verb in the Present Tense. This is the one where you need to add an "s" at the end. This is just one case under the rule called Subject-Verb Agreement. He/she/it runs very fast. Does she run fast? It looks great. How does it look? She cooks every day. How often does she cook? Oh yes, he loves her. Does he love her? I hope I managed to explain this and you can remember it, rather than just give you the right answer.

Did I tell you? Is correct

The question "did I tell you?" uses a present tense verb. A possible follow-up "do you understand?" also uses a present tense verb. When "have" is used though as in "have you studied these examples?", the verb is in the past tense.