Amit -

There are a few issues involved in your question.  As far as accessing data, you can access data in a Force.com object through our API, although I suspect you know that.

The larger issue has to do with recognizing duplicates companies.  The case that you cite does not really have a way for a computer to determine that the names are the same.  I guess you could write logic to check for all the cases, but that would probably not be worth the effort.

There are a couple of approaches you could take.  The first would be to assign a meaningful unique key, such as primary phone number.  Of course, users could have different primary phone numbers, either through multiple possibilities or simply typing mistakes.

You could also give users the opportunity to do a lookup on existing company names via a SOQL query.  For instance, the user could submit a query for companies with names like "test co" in your example, get the responses, and either select one of the values or enter a new company.  Once again, this depends on users properly searching and selecting the right company.

The Force.com platform does support the concept of an 'upsert', which updates a record if the unique key exists and inserts it if the unique key does not exist, but this is not the direction you want to move the data, nor will it solve the matching problems discussed.

Finally, you could have a custom object with the application or user name and the date and time last accessed, and you could select accounts added since that time.  Brute force, but it might get you where you want to go.

Hope this helps.