Quick warning: Luke 6:12-49 is not the same sermon as Matthew 5-7.   

1. Matthew devotes 3 long chapters where Luke is only one.
2. Matthew records 9 beatitudes where Luke only records 4.
3. Luke includes woe’s where Matthew does not at all.
4. Matthew is written more in third person (they) where Luke is more personal in second person (you).
5. In Matthew, Jesus goes up a mountain to teach, but in Luke, Jesus comes down to a level place in order to teach.

The above 5 points are only a few of the many difference between Luke and Matthew.  I point this out to simply say, we cannot read Luke and automatically think we know what it means because we read Matthew.  The gospel writers are different.  They have different purposes. What we discover here in Luke is that, while both sermons have overlapping content there are enough difference causing us to conclude they are different.