Well, I am new myself but I guess you got more than 1 slf4j package in your class path.
See the bold part below:
SLF4J API is desinged to bind with one and only one underlying logging framework at a time. If more than one binding is present on the class path, SLF4J will emit a warning, listing the location of those bindings. When this happens, select the one and only one binding you wish to use, and remove the other bindings.
For example, if you have both slf4j-simple-1.6.1.jar and slf4j-nop-1.6.1.jar on the class path and you wish to use the nop (no-operation) binding, then remove slf4j-simple-1.6.1.jar from the class path.
Note that the warning emitted by SLF4J is just that, a warning. SLF4J will still bind with the first framework it finds on the class path.