Repeating Sentence Start
Generally speaking, writers should vary their sentence starts. Otherwise, things become unreadable. Just imagine the following paragraph:
I went on vacation. I got lost on the way to the hotel. I got a key card but the key card didn't work, so I got another one. I liked my hotel room. I took a nap. I watched the football game after that.
When we start each sentence in the same way (in this case, "I" followed by a verb), we create monotony. It sounds a bit like a laundry list, doesn't it? And nobody wants to read a laundry list.
Thankfully, repeated sentence starts are pretty easy to spot. You'll likely catch them as you proofread. For additional help, you can also read your work aloud. If for some reason you don't see those repeated sentence starts, you'll likely hear them.
Of course, there are exceptions to every rule. For example, anaphora is a linguistic device in which the writer deliberately repeats sentence starts for literary effect. A classic example: "She did this. She did that. She did the other thing." As we can see, anaphora is best used to imply repetition. Just don't overdo it.