Consider the following verses from Mark 8.
And he cometh to Bethsaida; and they bring a blind man unto him, and besought him to touch him. And he took the blind man by the hand, and led him out of the town; and when he had spit on his eyes, and put his hands upon him, he asked him if he saw ought. And he looked up, and said, I see men as trees, walking. After that he put his hands again upon his eyes, and made him look up: and he was restored, and saw every man clearly. And he sent him away to his house, saying, Neither go into the town, nor tell it to any in the town.
- Mark 8:22-26 (KJV)
So why did Jesus have to heal this man twice? Could He not having done it in one turn?
This story needs to be seen "in context". Just a few verses up this story, Jesus says this.
Having eyes, see ye not? and having ears, hear ye not? and do ye not remember?
- Mark 8:18 (KJV)
I believe Jesus was merely saying: How often do I have to heal you, before you see clearly?
In other words, He used the "double" healing as an example to what was going on earlier.
A lesson for His disciples in other words.