A  topical collector generates these from millions of stamps they sort every year, often including some nice postally used!

Each pack of 250 stamps should have light duplication, and likely will also have some mixed condition. Note that generally because the source is a mixture, there normally are not many, if any complete sets.

When purchasing multiple 250 packs, purchasing up to 4 of the same type of topical pack is likely safe to maintain pretty good overall variety. The more lots purchased though, the higher the likelihood of overall duplication!

If you prefer to see exactly what stamps you will receive please search for the similar "topical hoard breakup airplanes, aircraft" lot listed individually in my eBay store. 

APS, CSDA, RPSC, & BNAPS member