Comparison of identical values found by LGTM.com
LGTM.com code analysis website found an interesting issue of comparison of identical values:

https://lgtm.com/projects/g/pandas-dev/pandas/snapshot/fe32cc08cf2e4dbdfa4aad33f3a74fada201600f/files/pandas/core/sparse/frame.py?sort=name&dir=ASC&mode=heatmap#L340

The line https://github.com/pandas-dev/pandas/blob/429078b01af47b356084c1da270b5b275fbf50f4/pandas/core/sparse/frame.py#L340

The `value == value` looks suspicious. It is there either by mistake or one was trying to check whether the value is float or not since as far as I know, the Boolean value of the comparison will always be the same, unless the value is the floating point value `float('nan')`.

If the latter, then there is a better way to do it with the `cmath.isinf(value) and not cmath.isnan(value)` to be explicit in the intention. However, maybe there is some bug in `numpy` or similar that you are trying to work around? @rockg @jreback 

I wanted to check with core developers before submitting a PR. 