The problem is, I have also seen many "saved" people go forward to the altar to get "saved." At youth camps and rallies I have seen some kids get "saved" every year. Now, of course there can be false professions. Paul says about these people that they have "believed in vain" (1 Corinthians 15:2). The people who made said professions may know in their hearts that there has never been a genuine faith in and acceptance of Christ for their salvation.
I would never tell someone who wants to get saved, "You are already saved, go sit down." But one must wonder why so many people make multiple professions. More specifically, why do so many people doubt their salvation? So, if you ever feel yourself doubting your salvation, I want to give you a few reasons I believe this might be happening:
1. You truly aren't saved.
With as much bad preaching that is out there in regards to salvation, it makes sense that many people are confused. It could be that someone keeps "getting saved" over and over because they have never clearly understood the gospel. It isn't because they "haven't repented enough," or they "haven't believed enough," or "there hasn't been an outward change." These are all confusing takes on the foundation of one's salvation. Salvation is based on the righteousness of God, and it is a "free gift" (plainly says so 3 times in Romans 5) through Jesus Christ.
2. You are living in sin and grieving the Holy Spirit.
The Bible makes it very clear that we all sin (even after our salvation). However, when we do so, we are grieving the Holy Spirit and He isn't as clearly identified in our hearts when He wants to communicate with us (Romans 8:16). A person who continues a life of sin, with little regard for God's will on the matter, is not likely to feel saved because they will not have that intimate relationship in which they would be constantly reminded of His presence.
3. You are not involved in evangelism.
Now, I'm in no way saying that you have to go soul-winning in order to be saved. There are many saved people who will bear very little fruit in their lives, yet they still possess eternal life. However, when you are involved in explaining the clear plan of salvation to other people, you are not only pleasing the Lord and walking in the Spirit, but you reminding yourself of why you know you are saved. The simple act of telling others will keep you from being deceived by those who "frustrate the grace of God" (Galatians 2:21), or by your own evil thoughts that would cause you to doubt your salvation.
I hope this helps. And if you are not sure what the "clear plan of salvation" is, please go to the SALVATION page on this website.