Bug introduced in 10.0 and fixed in 10.2 or earlier
ListLinePlot generates the same legend for different lists of data when PlotMarkers and PlotStyle options are used:
list = {{{1, 2}, {2, 3}, {3, 4}}, {{2, 4}, {4, 6}, {6, 8}}}; ListLinePlot[list, PlotStyle -> Black, PlotMarkers -> {Automatic, 16}, PlotLegends -> {"A", "B"}] 
Is this a bug?
I have Mathematica v10.0.0 installed on Windows 8 x64.
You just have to specify the LegendMarkers accordingly:
list = {{{1, 2}, {2, 3}, {3, 4}}, {{2, 4}, {4, 6}, {6, 8}}}; ListLinePlot[list, PlotStyle -> Black, PlotMarkers -> {Automatic, 16}, PlotLegends -> PointLegend[{"A", "B"}, LegendMarkers -> {Automatic, 16}]] This bug was fixed in Mathematica v10.3 or earlier.
$Version $\ $"10.3.0 for Microsoft Windows (64-bit) (October 9, 2015)"
list = {{{1, 2}, {2, 3}, {3, 4}}, {{2, 4}, {4, 6}, {6, 8}}}; ListLinePlot[list, PlotStyle -> Black, PlotMarkers -> {Automatic, 16}, PlotLegends -> {"A", "B"}] 
Another possibility is that you define your own markers:
markers = {Graphics[{Black, Disk[]}], Graphics[{Black, Rectangle[]}]}; list = {{{1, 2}, {2, 3}, {3, 4}}, {{2, 4}, {4, 6}, {6, 8}}}; ListPlot[list, PlotLegends -> SwatchLegend[{"A", "B"}, LegendMarkers -> markers, LegendLabel -> "Series", LegendFunction -> (Framed[#, RoundingRadius -> 5] &), LegendMargins -> 5], Joined -> True, PlotStyle -> Black, PlotMarkers -> Table[{size, 0.05}, {size, markers}]] 
