"'When I say to a wicked man, "You will surely die," and you do not warn him or speak out to dissuade him from his evil ways in order to save his life, that wicked man will die for his sin, and I will hold you accountable for his blood.'" NIV translation
If a watchman did not warn the people of approaching danger, then they would die and he would be the cause. As we see in this verse, the same is true if Ezekiel does not warn the people that judgment for their sins is coming. What if the lifeguard saw a bunch of sharks approaching a group of swimmers and did not warn them? We would all agree that their injuries and possible death would be because of the lifeguard. We, too, have that same responsibility (if we are in Christ). God places people in our path every day that need to hear the gospel and, if we do not share the good news that Jesus Christ paid for their sins, their blood is on our heads because we had the power to warn them but did not do so.
"'But if you do warn the wicked man and he does not turn from his wickedness or from his evil ways, he will die for his sin; but you will have saved yourself.'" NIV translation
Ezekiel's job was to proclaim the Word of God to the people and trust God to work in them to bring them to repentance.
The same is true for us today in that we are called to tell people the gospel of Jesus Christ but they must accept it and believe for themselves. Like Ezekiel, we must warn them that the penalty for sin is death and they can either accept the payment from Jesus or pay it for themselves.
If they reject the Truth then they will die for their sins. When God says "you will have saved yourself" it does not mean that we will lose our salvation if we do not tell someone about Jesus but it does mean that we will not have to answer for it in the judgment of saints. This is when Jesus hands out rewards for our service here on earth (see Revelation 20).
"'Again, when a righteous man turns from his righteousness and does evil, and I put a stumbling block before him, he will die. Since you did not warn him, he will die for his sin. The righteous things he did will not be remembered, and I will hold you accountable for his blood.'" NIV translation
Throughout the Bible, the number two is associated with testimony and the establishment of truth. When God repeats something, it is wise for us to pay attention. Here the concept of the responsibility of the watchman is repeated and so we, too, should take this to heart. The stumbling block that is mentioned here is grace through Jesus Christ and that causes those who trust in their own actions for salvation to stumble and fall. They can do a million things right but, without grace, one bad thing will lead to destruction.