CASE
WHEN [dca5_AeX AC Identification].[Hardware Serial Number] LIKE '%' THEN 'Computer'
END AS SMTYPE,
Can you replace this with this text:
'Computer' as 'SMTYPE'
It seems as though you just need this text to appear in a column, but don't care about the Hardware Serial Number value. I don't think this has anything to do with your WHERE clause but it's worth a guess.
Keep in mind DISTINCT is going to give you DISTINCT everything. If you have Bob from Ohio, Sue from Florida, Bob from Minnesota, and Eric from Oregon, SELECT DISTINCT FirstName will give you the results Bob, Sue, Eric. If you SELECT DISTINCT FirstName, State you're going to get Bob Ohio, Sue Florida, Bob Minnesota, Eric Oregon. You're selecting unique results from ALL columns. I could further extend this for them to have careers, and SELECT DISTINCT FirstName, State, Career you're going to get multiple Bobs from Minnesota since one is a plumber, one is a electrician, and one is a carpenter. (I guess you could call them builders.)
Your fix depends on what you're using this report for. Personally, I'd just drop MAC Address. This report is so massive I wonder what's needed on it. If it is needed, I'd add Inv_AeX_AC_TCPIP.Device so that it makes sense to the user why they're seeing duplicates for a single computer.
If you have computers with NULL Serial Numbers, have you tried changing your WHERE to 'IS NULL' instead of 'IS NOT NULL'? If these same NULL computers are listed, then reversing to IS NOT NULL should eliminate them. If different computers are listed, you know you may have something else going on -- like a blank value being reported as NULL, and you could try WHERE [Hardware Serial Number] <> ''
Does any of this help? It's tough to tell without having my hands data in a large environment this week.