For the last month or two I've been plagued by my Outlook contacts folder containing incomplete records, duplicates, missing email addresses, even as many as four versions of the same contact. This has been a real headache, and with the number of contacts reaching many many hundreds, fixing it by hand has been almost impossible.
The situation arose due to my change of employer. I exported my old contacts from outlook and re-imported them into my new instance - but even this process didn't work properly - some fields got lost, others mangled. It really was disappointing. On top of that, I hadn't realised that contacts I'd added from my old employer's Global Address list simply did not resolve to a set of complete details (most notably email addresses were missing).
To compound this, I then had the bright idea of trying to synchronise contacts across all my devices and domains - mobile phone, PDA, personal PC (using Outlook express) with PLAXO as the 'glue'. Sure, I got all my contacts in one place, but it created a mess.
For over a month I've been occasionally hand-fixing the odd record here and there, and looking on the web to find something to make the job easy. Amazingly Microsoft has no such tool to help - amazingly there is even no "find duplicates" function in Outlook, which alone would help helpful.
Then I found "duplicate killer" by 4team.
This wonderful bit of software is more powerful than its name suggests. As well as being able to identify and "kill" duplicates, perhaps more usefully it is able to merge records. You can do this fully automatically (identifying duplicates any way you choose, e.g. by names, by email etc.) or you can work through the gathered list of duplicates step-by-step, manually changing the fixes that the tool suggests. I have to say, so far I have found I am only having to change about 5% of the suggestions - but being the cautious type I am sticking with the manual preview.
The tool can also do similar tricks on your inbox and calendar - perhaps not something you would use quite as often, although non-the-less still useful (I do sometimes end up with duplicate appointments where both I and someone else create an appointment for the same meeting).
I thoroughly recommend "duplicate killer" if your Outlook contact database needs a good spring clean.