Combine Multiple Columns Into One Column With No Blank Spaces Or Duplicates

Hi, I would like to combine multiple columns into one column with no blank spaces or duplicates. I'm having difficulty starting the 2nd column as it seems to leave a gap when I try a formula with Index(), Rows(), and Countif() function. I'm thinking it's an offset or somehow. The second challenge is eliminating duplicates if that's possible. Attached an example spreadsheet of the data and what the result would look like